Introduction to Jython, Part 2: Programming essentials

About this tutorial

What is this tutorial about?

This is the second installment in a two-part tutorial designed to introduce you to the Jython scripting language. Jython is an implementation of Python that has been seamlessly integrated with the Java platform. Python is a powerful object-oriented scripting language used primarily in UNIX environments.

In Part 1 of this tutorial, you learned the basics of Jython, including installation and setup, access options and file compilation, syntax and data types, program structure, procedural statements, and functions. In Part 2 we will delve into some of the more advanced aspects of working with this powerful scripting language, starting with an in-depth introduction to object-oriented programming with Jython. We'll also discuss topics essential to the mechanics of application development in any language, including debugging, string processing, and file I/O.

By the time you have completed this second half of the two-part introduction to Jython, you will be able to write and implement complete functions, classes, and programs in Jython.

Should I take this tutorial?

This tutorial is designed as a progressive introduction to Jython. If you have not completed Part 1 of the tutorial, you should do so before proceeding to Part 2. Both the conceptual discussion and many of the code examples presented here will be difficult to follow without reference to Part 1.

In this second half of the tutorial,we will cover the following aspects of scripting with Jython:

  • Object-oriented programming with Jython
  • Debugging
  • Java support
  • String processing
  • File I/O
  • Building a Swing GUI application in Jython

To benefit from the discussion, you should be familiar with at least one procedural programming language and the basic concepts of computer programming, including command-line processing. To fully utilize Jython's features you should also be familiar with the basic concepts of object-oriented programming. To fully understand the GUI application example at the end of the tutorial you should have prior experience with Swing GUI programming, although you will be able to glean a lot from the preceding discussion and examples. It will also be helpful to have a working knowledge of the Java platform, because Jython runs on a JVM; although this is not a requirement of the tutorial.

Note that this tutorial is oriented towards Windows systems. All command examples will employ Windows syntax. In most cases similar commands perform the same functions on UNIX systems, although these commands will not be demonstrated.

Tools, code, and installation requirements

You must have Jython 2.1 or higher installed on your development system to complete this tutorial. Your development system may be any ASCII text editor (such as Windows Notepad) combined with the command prompt. The tutorial includes detailed instructions for getting and installing Jython on your system.

To use Jython you must also have a Java Runtime Environment (JRE) installed on your system. It is recommended that you use the latest JRE available (1.4.2 at the time of writing), but any version at or beyond Java 1.2 should work fine. If you are going to use Jython from a browser (that is, as an applet), you must have at least a JRE 1.1 available to the browser. See the Related topics section to download the latest version of the JDK.

All code examples in this tutorial have been tested on Jython running on the Sun Java 1.4.1 JRE on Windows 2000. Examples should work without change on any similar configuration on other operating systems.

Included with the tutorial is a set of appendices detailing all of the code examples you will use to learn about Jython. All code examples have been tested on Jython running on the Sun Java 1.4.1 JRE on Windows 2000. Examples should work without change on any similar configuration on other operating systems.

Object-oriented programming in Jython

A conceptual overview

Object-oriented programming (OOP) represents the state-of-the-art in software programming technique. OOP is based on the notion of creating a model (or simulation) of the target problem in your programs. Properly using OOP techniques reduces programming errors, speeds up software development, and facilitates the reuse of existing code. Jython fully supports the concepts and practice of OOP.

In the following sections I will introduce OOP and describe how it is achieved in Jython. In the next section I will discuss some of the more advanced features of object-oriented programming in Jython.

Objects in Jython

Jython is an object-oriented language that completely supports object-oriented programming. Objects defined by Jython have the following features:

  • Identity: Each object must be distinct and this must be testable. Jython supports the is and is not tests for this purpose.
  • State: Each object must be able to store state. Jython provides attributes (a.k.a. fields or instance variables) for this purpose.
  • Behavior: Each object must be able to manipulate its state. Jython provides methods for this purpose.

Note that the id(object) built-in function returns a unique integer identity value. So, the expression x is y is equivalent to id(x) == id(y).

OOP support in Jython

In its support for object-oriented programming, Jython includes the following features:

  • Class-based object creation: Jython classes are templates for the creation of objects. Objects are data structures with associated behavior.
  • Inheritance with polymorphism: Jython supports single- and multiple-inheritance. All Jython instance methods are polymorphic (or virtual) and may be overridden by subclasses.
  • Encapsulation with data hiding: Jython allows (but does not require) attributes to be hidden, thus permitting access outside the class itself only through methods of the class. Classes implement functions (called methods) to modify the data.

Defining a class

Defining a class is a lot like defining a module in that both variables and functions can be defined. Unlike the Java language, Jython allows the definition of any number of public classes per source file (or module). Thus, a module in Jython is much like a package in the Java language.

We use the class statement to define classes in Jython. The class statement has the following form:

class name ( superclasses ):  statement

  -- or --

class name ( superclasses ): 


When you define a class, you have the option to provide zero or more assignment statements. These create class attributes that are shared by all instances of the class. You can also provide zero or more function definitions. These create methods. The superclasses list is optional. We'll discuss superclasses a little later in the tutorial.

The class name should be unique in the same scope (module, function, or class). The class name is really a variable bound to the class body (similar to any other assignment). In fact, you can define multiple variables to reference the same class.

Creating a class instance

Classes are used to hold class (or shared) attributes or to create class instances. To create an instance of a class you call the class as if it were a function. There is no need to use a new operator like in C++ or the Java language. For example, with the class

class MyClass:

the following statement creates an instance:

x = MyClass()

Adding attributes to a class instance

In Jython (unlike in the Java language) clients can add fields (also known as attributes) to an instance. Only the one instance is changed. To add fields to an instance (x) just set new values on that instance, as shown below:

x.attr1 = 1
x.attr2 = 2
x.attrN = n

Defining class attributes and methods

Any variable bound in a class is a class attribute (or variable). Any function defined within a class is a method. Methods receive an instance of the class, conventionally called self, as the first (perhaps only) argument. For example, to define some class attributes and methods, you might enter:

class MyClass:
    attr1 = 10        # class attributes
    attr2 = "hello"

    def method1(self):
        print MyClass.attr1   # reference the class attribute

    def method2(self, p1, p2):
        print MyClass.attr2   # reference the class attribute

    def method3(self, text):
        self.text = text    # instance attribute
        print text, self.text    # print my argument and my attribute

    method4 = method3       # make an alias for method3

Note that inside a class, you should qualify all references to class attributes with the class name (for example, MyClass.attr1) and all references to instance attributes with the self variable (for example, self.text). Outside the class, you should qualify all references to class attributes with the class name (for example, MyClass.attr1) or an instance (for example, x.attr1) and all references to instance attributes with an instance (for example, x.text, where x is an instance of the class).

Hidden variables

To achieve data hiding, it is often desirable to create "private" variables, which can be accessed only by the class itself. Jython provides a naming convention that makes accessing attributes and methods outside the class difficult. If you declare names of the form: __xxx or __xxx_yyy (that's two leading underscores), the Jython parser will automatically mangle (that is, add the class name to) the declared name, in effect creating hidden variables. For example:

class MyClass:
    __attr = 10               # private class attribute

    def method1(self):

    def method2(self, p1, p2):

    def __privateMethod(self, text):
        self.__text = text    # private attribute

Note that unlike C++ and the Java language, all references to instance variables must be qualified with self; there is no implied use of this.

The init method

The __init__ method serves the role of an instance constructor. It is called whenever an instance is created. This method should be defined for all classes. Method __init__ may take arguments. In Jython, and unlike in C++ or the Java language, all instance variables (also known as attributes or fields) are created dynamically by assignment. They should be defined (that is, assigned to) inside __init__. This ensures they are defined for subsequent methods to use. Some examples are as follows:

class Class1:
    def __init__ (self):             # no arguments = []               # set implicit data

class Class2:
    def __init__ (self, v1, v2):     # 2 required arguments
        self.v1 = v1                 # set data from parameters
        self.v2 = v2

class Class3:
    def __init__ (self, values=None): # 1 optional argument
        if values is None: values = []
        self.values = values          # set data from parameter

The del method

If you allocate any resources in the __init__ method (or any other method), you need to ensure they are released before the object is deallocated. The best way to do this is by using the __del__ method. The __del__ method is called just before the garbage collector deallocates the object. You should also provide a cleanup method (typically named close, destroy, or dispose) that can be called directly. Here's an example:

class Class:
    def __init__ (self, db):    
        self.connection = db.getConnection()    # establish a connection

    def __del__ (self):                         # cleanup at death

    def close(self):                            # cleanup
        if not self.connection is None and self.connection.isOpen():
            self.connection.close()             # release connection
        self.connection = None

Using classes as values

Classes can also be assigned to variables (including function arguments). This makes writing dynamic code based on classes quite easy, as you can see from the following generic class instance factory:

def instanceMaker(xclass, *args):
    return apply(xclass, args)


x = instanceMaker(MyClass)   # same as: x = MyClass()


The ability to inherit from classes is a fundamental to object-oriented programming. Jython supports both single and multiple-inheritance. Single inheritance means there can be only one superclass; multiple inheritance means there can be more than one superclass.

Inheritance is implemented by subclassing other classes. These classes can be either other Jython classes or Java classes. Any number of pure-Jython classes or Java interfaces can be superclasses but only one Java class can be (directly or indirectly) inherited from. You are not required to supply a superclass.

Any attribute or method in a superclass is also in any subclass and may be used by the class itself or any client (assuming it is publicly visible). Any instance of a subclass can be used wherever an instance of the superclass can be used -- this is an example of polymorphism. These features enable reuse, rapid development, and ease of extension.

Below are some examples of inheritance:

class Class1: pass                # no inheritance

class Class2: pass

class Class3(Class1): pass        # single inheritance

class Class4(Class3,Class2): pass # multiple inheritance

from java import awt
from java import io         

# inherit a Java class and interface and a Jython class
class MyPanel(awt.Panel, io.Serializable, Class2): 

The init method with inheritance

The __init__ method of a subclass must call any __init__ method defined for its superclass; this is not automatic. The two examples below demonstrate how the __init__ method can be used with inheritance.

class Class1(SuperClass):
    def __init__ (self):               # no arguments
        SuperClass.__init__(self)      # init my super-class = []                 # set implicit data

class Class2(SuperClass):
    def __init__ (self, v1, v2):       # 2 required arguments
        SuperClass.__init__(self, v1)  # init my super-class with v1
        self.v2 = v2

And here are some examples of initializing with multiple inheritance:

class Class1(Super1, Super2):
    def __init__ (self):               # no arguments
        Super1.__init__(self)          # init each super-class
        Super2.__init__(self) = []                 # set implicit data

class Class2(Super1, Super2):
    def __init__ (self, v1, v2, v3):   # 3 required arguments
        # note you may do work before calling the super __init__ methods
        self.v3 = v3                   # set data from parameter
        Super1.__init__(self, v1)      # init each super-class
        Super2.__init__(self, v2)

Calling superclass methods

You can call any superclass method by qualifying it with the class name, as shown here:

class Class1:
    def method1 (self):             

class Class2(Class1):
    def method1 (self):         # override method1
        Class1.method1(self)    # call my super-class method

    def method2 (self):             

class Class3(Class2):
    def method1 (self):         # override method1
        Class2.method1(self)    # call my super-class method

    def method3 (self):             

Note that the secondary method definitions (in Class2 and Class3) override the superclass definitions. There is no requirement that the subclass method call its superclass method; however, if it doesn't, then it must completely replace the function of the superclass method.

Calling methods

There are two syntaxes for calling methods (assuming you have an instance of MyClass referenced by variable mci):

  • mci.someMethod(...)
  • MyClass.someMethod(mci, ...)

The first form typically is used in class client coding while the second one is used more often in subclasses to call superclass methods.

Advanced object-oriented programming

From theory to practice

In this section, we'll move from a conceptual overview of object-oriented programming in Jython to a more advanced discussion, incorporating topics such as operator overloading, special attributes, and introspection.

Special attributes

Jython classes provide support for several special attributes. The most significant are shown below:

__dict__The object's writeable attributesCan be used to introspect the attributes of an object
__class__The class of an objectAccess the class of the object (similar to x.getClass() in Java coding)
__bases__A tuple of the immediate superclasses of the objectCan be used to introspect the superclasses of the object

Changing the class of an existing instance

In Jython, unlike most other languages, you can change the class of an existing instance. Doing this changes the methods you can then use on the instance to the methods of the new class but not any of its pre-existing fields. For example, to change the class of an instance, assign the new class to the __class__ special attribute (see Special attributes), as shown below:

x = SomeClass()               
print isinstance(x, SomeClass)       # prints: 1 (true)
print isinstance(x, SomeOtherClass)  # prints: 0 (false)
# change the class (that is, the type) of the instance here
x.__class__ = SomeOtherClass  
print isinstance(x, SomeClass)       # prints: 0 (false)
print isinstance(x, SomeOtherClass)  # prints: 1 (true)

y = SomeOtherClass()               
print x.__class__ == y.__class__     # prints: 1 (true)

After this change, the x instance will support the methods of SomeOtherClass, not SomeClass as it did previously. Take care when changing the class of an object that the instance has the right attributes for the new class.

Introspecting attributes example

Here's a practical example using special attributes (see Special attributes). The module can introspect classes and instances to display their attributes and methods. I'll talk about introspection a little later, or you can check Introspection. You can also see String operations and functions and Appendix K: Built-in functions to learn more about the functions used below. For right now, just focus on the use of the callable function, the vars function (which implicitly uses the __dict__ attribute) and the __bases__ attribute.

__any__ = ['getMembers', 'printObject']

def addMember (list, item):
    if not item in list:

def getMembers (obj, memtype="attrs"):
    """ Get all the members (of memtype) of the object. """
    members = []
    for name, value in vars(obj).items():
            item = obj.__name__, name, value
            item = "<instance>", name, value
        if   memtype.lower().startswith("attr"):
            if not callable(value):
                addMember(members, item)
        elif memtype.lower().startswith("meth"):
            if callable(value):
               addMember(members, item)
        elif memtype.lower() == "all":
            addMember(members, item)
        for base in obj.__bases__:
            members.extend(getMembers(base, memtype))
    return members

import sys

def printObject (obj, stream=sys.stdout):
    """ Print all the members of the object. """
    members = getMembers(obj, "attrs")
    print >>stream, "Attributes:"
    for objname, memname, value in members:
        print >>stream, "  %s.%s" % (objname, memname)

    members = getMembers(obj, "methods")
    print >>stream, "Methods:"
    for objname, memname, value in members:
        print >>stream, "  %s.%s" % (objname, memname)

Introspecting attributes example testcase

The following code uses the functions in the previous section to introspect the UserList class. See Operator overloading for the definition of the UserList class.

if __name__ == "__main__":

    from UserList import UserList
    class MyClass(UserList):
        def __init__ (self, x, y):
            self.__x = x
            self.__y = y
        def method1 (self):
            return self.x + self.y
        def method2 (self, x, y):
            return  self.x + self.y + x + y

    print "For class:", `MyClass`

    aMyClass = MyClass(1, 2)
    print "For instance:", `aMyClass`

Output of get members

The following output (reformatted into multiple columns to save space) is the result of running the main code from the above module. Notice that the private fields and methods (see Hidden variables) have mangled names.

For class: <class __main__.MyClass at 28921555>
Attributes:            Methods:                     
  MyClass.__doc__        MyClass.__init__         UserList.__len__            
  MyClass.__module__     MyClass.method1          UserList.__lt__             
  UserList.__doc__       MyClass.method2          UserList.__mul__            
  UserList.__module__    UserList._UserList__cast UserList.__ne__             
                         UserList.__add__         UserList.__radd__           
                         UserList.__cmp__         UserList.__repr__           
                         UserList.__contains__    UserList.__rmul__           
                         UserList.__delitem__     UserList.__setitem__        
                         UserList.__delslice__    UserList.__setslice__       
                         UserList.__eq__          UserList.append             
                         UserList.__ge__          UserList.count              
                         UserList.__getitem__     UserList.extend             
                         UserList.__getslice__    UserList.index              
                         UserList.__gt__          UserList.insert             
                         UserList.__iadd__        UserList.pop                
                         UserList.__imul__        UserList.remove             
                         UserList.__init__        UserList.reverse            
                         UserList.__le__          UserList.sort               

For instance: [1, 2, 3, 4]


Note that methods and class attributes reside with classes and instance attributes reside with instances. Yet all the class's methods can be applied to each instance.


You will often need to determine, at runtime, the characteristics of an object. We call this introspecting the object. The Java platform offers introspection services via the java.lang.Class class and classes in the java.lang.reflect package. While powerful, these APIs are somewhat difficult to use. As you probably already suspected, Jython offers a simpler approach to introspection.

In Jython, we can use the dir and vars functions to examine the bindings for any object, such as modules, functions, classes, sequences, maps, and more. To better understand how this works, consider the following example. The output has been inserted (and reformatted) after the print statements prefixed with "..." for easier reading. The dir function returns only the binding names, while the vars function returns the names and values; thus, when the same names are returned by both functions, we need use only the vars function, as shown below:

#-- empty start --
print "vars:", vars()
...vars: {'__doc__': None, '__name__': '__main__'}

x = 1
y = 2
z = 3
l = [x, y, z]
d = {x:"xxxx", y:"yyyy", z:"zzzz"}

#-- locals variables --
print x, y, z, l, d
...1 2 3 [1, 2, 3] {3: 'zzzz', 2: 'yyyy', 1: 'xxxx'}

#-- plus locals variables --
print "vars:", vars()
...vars: {'__name__': '__main__', 'x': 1, \
... 'd': {3: 'zzzz', 2: 'yyyy', 1: 'xxxx'}, '__doc__': None, \
... 'y': 2, 'l': [1, 2, 3], 'z': 3}

import sys

#-- plus import --
print "vars:", vars()
...vars: {'__name__': '__main__', 'z': 3, 'l': [1, 2, 3], \
... '__doc__': None, 'y': 2, 'x': 1, 'sys': sys module, \
... 'd': {3: 'zzzz', 2: 'yyyy', 1: 'xxxx'}}

#-- sys import --
print "vars sys:", vars(sys)  
...vars sys: {'classLoader': \
...     <beanProperty classLoader type: java.lang.ClassLoader at 31845755>, 
...     ... many values removed ...,
... 'warnoptions': <reflected field public static \
...     org.python.core.PyList \
...     org.python.core.PySystemState.warnoptions at 1024901>}

del x, y, z

#-- post delete --
print "vars:", vars()
...vars: {'__name__': '__main__', 'l': [1, 2, 3], '__doc__': None, \
... 'sys': sys module, 'd': {3: 'zzzz', 2: 'yyyy', 1: 'xxxx'}}

def func (x, y):
    return x, y

class MyClass ():
    def __init__ (self, x, y):
        self.__x = x
        self.__y = y

    def method1 (self):
        return self.x + self.y

    def method2 (self, x, y):
        return  self.x + self.y + x + y

#-- plus function and class --
print "vars:", vars()
....vars: {'func': <function func at 21569784>, '__name__': '__main__', \
...  'l': [1, 2, 3], '__doc__': None, \
.... 'MyClass': <class __main__.MyClass at 1279942>, \
...  'sys': sys module, 'd': {3: 'zzzz', 2: 'yyyy', 1: 'xxxx'}}

#-- function --
print "dir: ", dir(func)     # **** dir and vars different here ****
print "vars:", vars(func)
...dir:  ['__dict__', '__doc__', '__name__', 'func_closure', \
... 'func_code', 'func_defaults', 'func_doc', 'func_globals', 'func_name']
...vars: None

#-- class --
print "vars:", vars(MyClass)
...vars: {'__doc__': None, '__init__': <function __init__ at 17404503>, \
... 'method2': <function method2 at 23511968>, '__module__': '__main__', \
... 'method1': <function method1 at 28670096>}

myclass = MyClass(1, 2)

#-- instance --
print "myclass:", myclass
print "vars:", vars(myclass)
...myclass: <__main__.MyClass instance at 19014134>
...vars: {'_MyClass__y': 2, '_MyClass__x': 1}

Note that dir(x) is generally equivalent to x.__dict__.keys() and vars(x) is generally equivalent to x.__dict__.

Additional functions for introspection

The attributes described in Special attributes allow additional introspection of classes. In particular you can use the __dict__ attribute to determine the methods in a class and the fields in an instance.

In addition to dir and vars, Jython provides several more functions for introspecting classes and instances, as follows:

hasattr(obj, name)Tests to see if the named attribute exists
getattr(obj, name {, default})Gets the named attribute if it exists; else default is returned (or an exception is raised if no default is provided)
setattr(obj, name, value)Sets the named attribute's value
delattr(obj, name)Removes the named attribute

See Appendix K: Built-in functions to learn more about these functions.

Abstract classes

Abstract classes are classes in which some or all of the methods are missing or have incomplete definitions. A subclass must be created to provide or complete these method definitions. Concrete classes are not abstract (that is, all the methods are complete). So far we have been working only with concrete classes. Abstract classes are created to facilitate reuse. They provide a partial implementation of a design that you can complete or extend by subclassing them.

To get a better understanding of how this works, we will create a simple abstract command framework that supports command do, undo, and redo actions. Commands are defined in (sub)classes and can be added easily by creating new do_... and undo_... methods. We access these methods via introspection, as discussed in the previous sections.

An abstract command framework

Here's the example abstract command framework:

class CommandProcessor:   # an abstract class
    """ Process Commands. """

    def __init__ (self):
        self.__history = []
        self.__redo = []

    def execute (self, cmdName, *args):
        """ Do some command """
        self.__history.append( (cmdName, args) )
        processor = getattr(self, "do_%s" % cmdName, None)
        if processor:
            return processor(*args)
            raise NameError, "cannot find do_%s" % cmdName

    def undo (self, count=1):
        """ Undo some (or all) commands in LIFO order """
        self.__redo = []
        while count > 0 and len(self.__history) > 0:
            cmdName, args = self.__history.pop()
            count -= 1
            processor = getattr(self, "undo_%s" % cmdName, None)
            if processor:
                self.__redo.append( (cmdName, args) )
                raise NameError, "cannot find undo_%s" % cmdName

    def redo (self, count=1):
        """ Redo some (or all) undone commands """
        while count > 0 and len(self.__redo) > 0:
            cmdName, args = self.__redo.pop()
            count -= 1
            processor = getattr(self, "do_%s" % cmdName, None)
            if processor:
                raise NameError, "cannot find do_%s" % cmdName

Note:This example is based on code from Jython Essentials by Samuele Pedroni and Noel Rappin (see Related topics for more information).

A test case for the example framework

Here's a test case for the example abstract command framework:

class MyProcessor (CommandProcessor):  # a concrete subclass
    def __init__ (self):

    def do_Cmd1 (self, args):
        print "Do Command 1:", args

    def do_Cmd2 (self, args):
        print "Do Command 2:", args

    def do_Cmd3 (self, args):
        print "Do Command 3:", args

    def undo_Cmd1 (self, args):
        print "Undo Command 1:", args

    def undo_Cmd2 (self, args):
        print "Undo Command 2:", args

    def undo_Cmd3 (self, args):
        print "Undo Command 3:", args

mp = MyProcessor()

print "execute:" ; mp.execute("Cmd1", None)
print "execute:" ; mp.execute("Cmd2", (1,2,3))
print "execute:" ; mp.execute("Cmd3", "Hello")
print "undo:   " ; mp.undo(2)
print "redo:   " ; mp.redo(2)

print "execute:", ;mp.execute("BadCmd", "Hello")

The framework with the given test case produces the following output:

Do Command 1: None
Do Command 2: (1, 2, 3)
Do Command 3: Hello
Undo Command 3: Hello
Undo Command 2: (1, 2, 3)
Do Command 2: (1, 2, 3)
Do Command 3: Hello
Traceback (innermost last):
  File "", line 63, in ?
  File "", line 15, in execute
NameError: cannot find do_BadCmd

Operator overloading

Like C++, but unlike the Java language, Jython allows many of the standard language operators to be overloaded by classes. This means classes can define a specific meaning for the language operators. Jython also allows classes to emulate built-in types like numbers, sequences, and maps. To learn more about emulation see Appendix B: Common overloaded operators and methods.

In the example that follows, we'll use the standard Jython UserList class definition to show an example of operator overloading in practice. UserList is a class that wraps a list and behaves as a list does. Most of its function is delegated (passed on to) its contained list, called data. In a more realistic example, these overloaded functions would be implemented to access some other store, such as a disk file or a database.

class UserList:
    def __init__(self, initlist=None): = []
        if initlist is not None:
            if   type(initlist) == type(
      [:] = initlist
            elif isinstance(initlist, UserList):
      [:] =[:]
       = list(initlist)

    def __cast(self, other):
        if isinstance(other, UserList): return
        else:                           return other

    #  `self`, repr(self)
    def __repr__(self): return repr(

    #  self < other
    def __lt__(self, other): return <  self.__cast(other)

    #  self <= other
    def __le__(self, other): return <= self.__cast(other)

    #  self == other
    def __eq__(self, other): return == self.__cast(other)

    #  self != other, self <> other
    def __ne__(self, other): return != self.__cast(other)

    #  self > other
    def __gt__(self, other): return >  self.__cast(other)

    #  self >= other
    def __ge__(self, other): return >= self.__cast(other)

    #  cmp(self, other)
    def __cmp__(self, other):
        raise RuntimeError, "UserList.__cmp__() is obsolete"

    #  item in self
    def __contains__(self, item): return item in

    #  len(self)
    def __len__(self): return len(

    #  self[i]
    def __getitem__(self, i): return[i]

    #  self[i] = item
    def __setitem__(self, i, item):[i] = item

    #  del self[i]
    def __delitem__(self, i): del[i]

    #  self[i:j]
    def __getslice__(self, i, j):
        i = max(i, 0); j = max(j, 0)
        return self.__class__([i:j])

    #  self[i:j] = other
    def __setslice__(self, i, j, other):
        i = max(i, 0); j = max(j, 0)
        if   isinstance(other, UserList):
  [i:j] =
        elif isinstance(other, type(
  [i:j] = other
  [i:j] = list(other)

    #  del self[i:j]
    def __delslice__(self, i, j):
        i = max(i, 0); j = max(j, 0)

    #  self + other   (join)
    def __add__(self, other):
        if   isinstance(other, UserList):
            return self.__class__( +
        elif isinstance(other, type(
            return self.__class__( + other)
            return self.__class__( + list(other))

    #  other + self   (join)
    def __radd__(self, other):
        if   isinstance(other, UserList):
            return self.__class__( +
        elif isinstance(other, type(
            return self.__class__(other +
            return self.__class__(list(other) +

    #  self += other  (join)
    def __iadd__(self, other):
        if   isinstance(other, UserList):
        elif isinstance(other, type(
   += other
   += list(other)
        return self

    #  self * other   (repeat)
    def __mul__(self, n):
        return self.__class__(*n)
    __rmul__ = __mul__

    #  self *= other  (repeat)
    def __imul__(self, n): *= n
        return self

    # implement "List" functions below:

    def append(self, item):

    def insert(self, i, item):, item)

    def pop(self, i=-1): return

    def remove(self, item):

    def count(self, item): return

    def index(self, item): return

    def reverse(self):

    def sort(self, *args): apply(, args)

    def extend(self, other):
        if isinstance(other, UserList):

Nested classes

Like functions, classes can be nested. Nested classes in Jython work similarly to static inner classes in the Java language. Here's an example:

class MyDataWrapper:
    class Data: pass    # inner data structure class

    def __init__ (self): = Data()

    def set (self, name, value):
        setattr(, name, value)

    def get (self, name, default=None):
        return getattr(, name, default)

Debugging Jython

Using print statements for debugging

Like any programming language, Jython supports the use of print statements for debugging. To implement this debugging solution, we simply add a print statement to a program, run the program, and examine the generated output for clues to the bugs. While very basic, this debugging solution is in many cases completely satisfactory.

Here's an example print statement for debugging.

def myFunc(x):
    print "x at entry:", x
    print "x at exit:", x
    return x

z = myFunc(20)

The Jython debugger

For the times when the print-statement solution isn't sufficient for your debugging needs, Jython provides a simple, command-line debugger similar to the jdb debugger for the Java platform. The Jython debugger is written entirely in Jython and can thus be easily examined or extended. In addition, Jython provides a set of abstract base debugging classes to allow other debuggers, such as a GUI debugger, to be built on this framework.

To launch the debugger run the following command:

c:\>jython c:\jython-2.1\lib\ <test_module>.py

An example Jython debugging session

Debugger commands are enterred after the debugger prompt "(Pdb)." Here's an example debugging session using the module (see The factorial engine:

C:\Articles>jython \jython-2.1\lib\
> C:\Articles\<string>(0)?()
(Pdb) step
> C:\Articles\<string>(1)?()
(Pdb) step
> C:\Articles\
(Pdb) list 67
 62             try:
 63                 print "For", value, "result =", 
 64             except ValueError, e:
 65                 print "Exception -", e
 67         doFac(-1)
 68         doFac(0)
 69         doFac(1)
 70         doFac(10)
 71         doFac(100)
 72         doFac(1000)
(Pdb) tbreak 67
Breakpoint 1 at C:\Articles\
(Pdb) continue running...
Deleted breakpoint 1
> C:\Articles\
-> doFac(-1)
(Pdb) next
For -1 result = Exception - only positive integers supported: -1
> C:\Articles\
-> doFac(0)
(Pdb) next
For 0 result = 1
> C:\Articles\
-> doFac(1)
(Pdb) next
For 1 result = 1
> C:\Articles\
-> doFac(10)
(Pdb) next
For 10 result = 3628800
> C:\Articles\
-> doFac(100)
(Pdb) next
For 100 result = 
> C:\Articles\
-> doFac(1000)
(Pdb) next
For 1000 result = 402387260077 ... many other digits deleted ...
> C:\Articles\>None
-> doFac(1000)
(Pdb) next
> C:\Articles\<string>(1)?()->None
(Pdb) next

To learn more about debugging with the Jython debugger, see Appendix C: Jython debugger commands.

Jython profiler

Sometimes you may notice that a Jython program runs longer than you expect. You can use the Jython profiler to find out what sections of the program take the longest time and optimize them. The profiler will let you profile entire programs or just individual functions.

Here's an example run, profiling the program (see The factorial engine:

c:\>jython \jython-2.1\lib\ \articles\

\articles\ running...
For -1 result = Exception - only positive integers supported: -1
For 0 result = 1
For 1 result = 1
For 10 result = 3628800
For 100 result = 
For 1000 result = 402387260077 ... many other digits deleted ...

         237 function calls (232 primitive calls) in 0.250 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.130    0.130    0.240    0.240 <string>:0(?)
        1    0.000    0.000    0.110    0.110
      220    0.010    0.000    0.010    0.000 \
        6    0.060    0.010    0.070    0.012
        1    0.000    0.000    0.000    0.000
        1    0.000    0.000    0.000    0.000
      6/1    0.040    0.007    0.110    0.110
        1    0.010    0.010    0.250    0.250 \
        0    0.000             0.000          profile:0(profiler)

From this run you can see that (besides the initial startup code) most of the program time is being used by the calculate function. For more information on profiling Jython see the Python Reference Manual , available in Related topics.


Like C and the Java language (as of version 1.4), Jython supports assertions. Assertions are conditions that must be true for the program to work correctly; if they are not true the program may behave unpredictably. Often they are used to validate input values to functions. Jython's support for assertions comes in the form of the following assert statement:

assert expression {, message}

Note that expression is any Jython expression; if it is false an exceptions.AssertionError exception is raised. If message is provided, it becomes the message associated with the exception. For example:

def myFunc(x):
    assert x >= 0, "argument %r must be >= 0" % x
    return fac(x)
z = myFunc(20)          # no exception raised

z = myFunc(-1)          # AssertionError raised

Java support in Jython

Using Java services in Jython code

One of Jython's most powerful features is its ability to interface with Java code. A Jython program can create instances of any Java class and call any method on any Java instance. Jython can also subclass Java classes, allowing Java code to call Jython code. Jython makes calling Java methods very easy by making strong but transparent use of the Java Reflection API (package java.lang.reflect).

To complete this section of the tutorial, you need to be familiar with the Java language and select Java runtime APIs. You should understand the basic notions of object-oriented programming on the Java platform, as well as being familiar with the Java data types, classes, threads, and the services in the java.lang, java.util, and javax.swing packages.

Note:Because the reflection APIs have been highly optimized in version 1.4, Jython runs much faster on Java version 1.4 and above.

Calling Jython from Java code

As shown in Inheritance, a Jython class can subclass Java classes. Subclassing makes it very easy to extend Java classes (such as GUI components). This allows Java code to call Jython code without realizing it is Jython code. It also makes it possible to implement in Jython classes used by other Java code, as shown in the following example:

from java import util
class MyArray(util.ArrayList):  # subclass a Java class
    def get (self, index):      # override the get method
        "@sig public java.lang.Object get(int index)"
      if 0 <= index < self.size:  
            return util.ArrayList.get(self, index)
        return None             # OutOfBounds now returns null

After being compiled by jythonc the above class can be used in Java code anywhere an java.util.ArrayList instance can be used. Note that when calling a superclass method, the self value is passed as an argument.

Calling Java classes from Jython

In addition to subclassing Java classes it is also possible to access Java classes directly in Jython. For example, this code sequence:

from java.util import Date


d = Date()   # now
print d, d.time, d.getTime()

will produce the following output:

Tue Dec 02 14:44:02 CST 2003 1070397842496 1070397842496

Using JavaBean properties from Jython

In the example from Calling Java classes from Jython you may have noticed that the expressions d.time and d.getTime() produce the same result. This is because they do the same thing. Jython has a very convenient feature that makes JavaBean properties appear as Jython attributes. JavaBean properties are defined by (typically) matching pairs of Java methods of the following form, where <type> is the type of the property and <name> is the name of the property.:

<type> get<name>()

-- and --

void set<name>(<type> value)

For example the Java methods long getTime() { ... } and void setTime(long t) { ... } define the long property time. Thus a Jython reference d.time is automatically and dynamically converted into the Java expression d.getTime().

Jython can also set properties, thus d.time = 1000000L is allowed. The Jython reference d.time = value is automatically and dynamically converted into the Java expression d.setTime(value). Once this change is applied, the print statement from Calling Java classes from Jython results in the following:

Wed Dec 31 18:01:40 CST 1969 100000 100000

Calling methods on Java objects

It is very easy to call methods on Java objects; just call them like they are Jython methods. Jython automatically maps parameter and return values to and from Jython and Java types. For example, here is a short sequence of Jython that uses Java classes and methods extensively:

1: from javax import swing 
2: import sys
4: f = swing.JFrame(sys.argv[1], size=(200,200), 
5:                  defaultCloseOperation=swing.JFrame.EXIT_ON_CLOSE)
6: f.contentPane.add(swing.JLabel(sys.argv[2]))
7: f.visible = 1

This code sequence creates and shows a GUI frame window. The script's first command-line argument becomes the title and the second the content text. Line 4 creates the frame, passing in the title, the desired size, and a close action. The size and defaultCloseOperation parameters are properties as described above and, as such, may be (quite conveniently) set in the JFrame's constructor when invoked from a Jython program. The title is set as a parameter of the JFrame's equivalent of the __init__ method. Line 6 accesses the JFrame's contentPane property and calls its add method to add a JLabel to show the second argument. Line 7 makes the frame visible by setting its visible property to 1 (true).

A sample of this GUI is shown below:

Overriding Java methods and properties

As shown in Calling Jython from Java code, when overriding Java methods in classes that can be called from the Java language, you need to provide signature information. This is done via documentation comments. The first line of the comment, if it starts with "@sig", is used as a directive to the jythonc program (discussed in Part 1) to generate a Java-compatible method signature. For example, the comment below describes the get method using the Java language's declaration syntax. In signatures types must be fully qualified.

"@sig public java.lang.Object get(int index)"

Jython does not support overloaded methods, which are methods with the same name but with differing number and/or types of arguments. Instead, Jython supports defaulted arguments and variable number of arguments, which can create a problem if you inherit from a Java class that uses overloading and you want to override the overloaded methods. In Jython, you must define the base method and accept a varying number of arguments. Consider the (rather impractical) example of an InputStream that always returns a blank:

from java import io

class AlwaysBlank(io.InputStream):
    # covers all forms of read(...)
    def read(self, *args):
        if len(args) > 0: 
            # covers forms: int read(byte[])
            #               int read(byte[], int off, int len)
            return apply(, (self,) + args)
            # covers form: int read()
            return ord(' ')

This code is based on an example from the Jython home page.

Java arrays from Jython

Jython supports the creation of Java-style array objects. Arrays are used primarily to pass arrays to and return arrays from Java methods, but they are general purpose and can be used in pure Jython code. Array elements are typed using Java base and class types. Arrays act much like Jython lists but they cannot change length.

Array support is provided by the jarray module. The two functions in the jarray module, zeros and array, are used to create arrays. The array function maps a Jython sequence to a Java array. Some examples are as follows:

from jarray import zeros, array
from java import util
from javax import swing

a1 = zeros(100, 'i')            # an array of 100 int 0s
a2 = array([1,2,10,-5,7], 'i')  # an array of ints as listed

# an array of doubles 0.0 to 49.0
a3 = array([i * 1.0 for i in range(50)], 'd')  

a4 = zeros(10, util.Map)        # an array of 10 null Maps
a5 = array((swing.JFrame("F1"), # an array of 3 JFrames
            swing.JFrame("F3")), swing.JFrame)
a6 = array("Hello", 'c')        # an array of characters

See Appendix A: Character codes for array types for a listing of character codes for array types.

Java thread support in Jython

Java threads

The Java runtime makes extensive use of threads, which it uses to handle GUI events, to perform asynchronous I/O, to implement asynchronous processing, and so on.

It's easy to create Java threads in Jython: just create instances of java.lang.Thread and subclasses of java.lang.Runnable. For an example, see The GUI: You can also create threads out of Jython functions by using the thread module and functions of the following form:

start_new_thread(function, args)

-- and --


The start_new_thread function runs the function argument in a new Java thread, passing the args tuple value to the function. The exit function can be used in the thread to end it (generally as the target of an if statement).

Java synchronization

When developing multithreaded programs using Java or Jython threads, it is sometimes necessary to create synchronized functions (or methods). Synchronized functions are functions that can only be called from one thread at a time; meaning that other threads are prevented from entering the function until the first thread exits. Jython provides the synchronized module and two functions to create synchronized functions. The functions are of the following form:


-- and --

apply_synchronized(syncobj, function, pargs {, kwargs})

The make_synchronized function permanently synchronizes the function argument. The apply_synchronized function temporarily synchronizes on syncobj and then calls the function argument.

Example: Using make_synchronized

Using make_synchronized to signal events is quite straightforward, as shown below:

from synchronize import *
from java import lang

# define synchronization helpers

def waitForSignal (monitor):
    """ Wait until the monitor is signaled. """
# replace with synchronized version; syncs on 1st argument
waitForSignal = make_synchronized(waitForSignal)

def notifySignal (monitor):
    """ Signal monitor. """
# replace with synchronized version; syncs on 1st argument
notifySignal = make_synchronized(notifySignal)

class Gui:           # GUI support
    def doExit (self):
        self.visible = 0

if __name__ == "__main__":     # main code
    gui = Gui()
    print "Waiting until GUI exit requested..."
    print "Done"

A Jython threading example

Here's an example of the use of Jython threads. The example shows a set of producer and consumer threads sharing access to a common buffer. We'll start with the definition of the shared buffer, as shown below.

""" A Jython Thread Example. """

from java import lang 
from synchronize import *
from thread import start_new_thread
from sys import stdout

def __waitForSignal (monitor):
    apply_synchronized(monitor, lang.Object.wait, (monitor,))

def __signal (monitor):
    apply_synchronized(monitor, lang.Object.notifyAll, (monitor,))

def __xprint (stream, msg):
    print >>stream, msg

def xprint (msg, stream=stdout):
    """ Synchronized print. """
    apply_synchronized(stream, __xprint, (stream, msg))

class Buffer:
    """ A thread-safe buffer. """

    def __init__ (self, limit=-1):
        self.__limit = limit    # the max size of the buffer
        self.__data = []
        self.__added = ()       # used to signal data added
        self.__removed = ()     # used to signal data removed

    def __str__ (self):
        return "Buffer(%s,%i)" % (self.__data, self.__limit)

    def __len__ (self):
        return len(self.__data)

    def add (self, item):
        """ Add an item. Wait if full. """
        if self.__limit >= 0:
            while len(self.__data) > self.__limit:
        xprint("Added: %s" % item)

    def __get (self):
        item = self.__data.pop(0)
        return item

    def get (self, wait=1):
        """ Remove an item. Wait if empty. """
        item = None
        if wait:
            while len(self.__data) == 0:
            item = self.__get()
            if len(self.__data) > 0: item = self.__get()
        xprint("Removed: %s" % item)
        return item
    get = make_synchronized(get)

Producer and consumer definitions

The next step in the example is to take a look at the producer and consumer definitions, shown here:

class Producer:
    def __init__ (self, name, buffer):
        self.__name = name
        self.__buffer = buffer

    def __add (self, item):

    def __produce (self, *args):
        for item in args:

    def produce (self, items):
        start_new_thread(self.__produce, tuple(items))

class Consumer:
    def __init__ (self, name, buffer):
        self.__name = name
        self.__buffer = buffer

    def __remove (self):
        item = self.__buffer.get()
        return item

    def __consume (self, count):
        for i in range(count):

    def consume (self, count=1):
        start_new_thread(self.__consume, (count,))

An trial run of the threading example

And finally, here's a trial run of the example code:

# all producers and consumer share this one
buf = Buffer(5)    

p1 = Producer("P1", buf)
p2 = Producer("P2", buf)
p3 = Producer("P3", buf)
p4 = Producer("P4", buf)
c1 = Consumer("C1", buf)
c2 = Consumer("C2", buf)

# create 6 items
p1.produce(["P1 Message " + str(i) for i in range(3)])
p2.produce(["P2 Message " + str(i) for i in range(3)])

# consume 20 items
for i in range(5):

# create 20 more items
p3.produce(["P3 Message " + str(i) for i in range(10)])
p4.produce(["P4 Message " + str(i) for i in range(10)])

# consume 4 items

# let other threads run

xprint("Buffer has %i item(s)left" % len(buf))

Output of the example

The producer consumer example produces the following results (wrapped to two columns to save space):

Added: P1 Message 0       Added: P3 Message 7       
Added: P1 Message 1       Removed: P3 Message 7     
Added: P1 Message 2       Added: P3 Message 8       
Added: P2 Message 0       Removed: P3 Message 8     
Added: P2 Message 1       Added: P3 Message 9       
Added: P2 Message 2       Removed: P3 Message 9     
Removed: P1 Message 0     Added: P4 Message 0       
Removed: P1 Message 1     Removed: P4 Message 0     
Removed: P1 Message 2     Added: P4 Message 1       
Removed: P2 Message 0     Removed: P4 Message 1     
Removed: P2 Message 1     Added: P4 Message 2       
Removed: P2 Message 2     Removed: P4 Message 2     
Added: P3 Message 0       Added: P4 Message 3       
Removed: P3 Message 0     Removed: P4 Message 3     
Added: P3 Message 1       Added: P4 Message 4       
Removed: P3 Message 1     Added: P4 Message 5       
Added: P3 Message 2       Added: P4 Message 6       
Removed: P3 Message 2     Added: P4 Message 7       
Added: P3 Message 3       Added: P4 Message 8       
Removed: P3 Message 3     Added: P4 Message 9       
Added: P3 Message 4       Removed: P4 Message 4     
Removed: P3 Message 4     Removed: P4 Message 5     
Added: P3 Message 5       Removed: P4 Message 6     
Removed: P3 Message 5     Removed: P4 Message 7     
Added: P3 Message 6       Buffer has 2 item(s)left  
Removed: P3 Message 6

Interfacing with Java services

Creating the interface

Often you will need to use Java services from within Jython code. In these cases, you can either do it openly each time you need to use a given service, or you can wrap the Java services in a Jython library function and use that function in your Jython code.

The second option is recommended because it encapsulates and abstracts the Java code.

Wrapping Java services in Jython

As an example of how you might wrap a Java service in a Jython library function, we'll take a look at the module excerpts. The JavaUtils module is introduced by the code below. See Part 1 of this tutorial to refresh your memory about modules.

""" This module defines several functions to ease interfacing with Java code."""

from types import *

from java import lang
from java import util
from java import io

# only expose these
__all__ = ['loadProperties', 'getProperty', 
           'mapToJava', 'mapFromJava', 'parseArgs']

Accessing Java properties files

You will often need to access Java properties files to get configuration information. Jython lets you use the loadProperties and getProperty functions for this, as shown below:

def loadProperties (source):
    """ Load a Java properties file into a Dictionary. """
    result = {}
    if type(source) == type(''):    # name provided, use file
        source = io.FileInputStream(source)
    bis = io.BufferedInputStream(source)
    props = util.Properties()
    for key in props.keySet().iterator():
        result[key] = props.get(key)
    return result

def getProperty (properties, name, default=None):
    """ Gets a property. """
    return properties.get(name, default)

Properties file example

So, for example, if you were to use the functions from Accessing Java properties files as shown below

import sys
file = sys.argv[1]
props = loadProperties(file)
print "Properties file: %s, contents:" % file 
print props
print "Property %s = %i" % ('debug', int(getProperty(props, 'debug', '0')))

with the properties file content of

# This is a test properties file
debug = 1
error.level = ERROR = false

then the resulting output would be:

Properties file:, contents:
{'error.level': 'ERROR', 'debug': '1', '': 'false'}
Property debug = 1

Mapping Java types

Sometimes you need to create pure-Java objects in Jython (for example, when you need to create objects to send across a network to a Java-based server, or when you need to pass the object to a type-sensitive Java service, such as with Swing table cell values). To convert Jython built-in types to Java types (and vice versa) use the functions in the following example (a continuation of the module excerpt from Wrapping Java services in Jython):

def mapMapFromJava (map):
    """ Convert a Map to a Dictionary. """
    result = {}
    iter = map.keySet().iterator()
    while iter.hasNext():
        key =
        result[mapFromJava(key)] = mapFromJava(map.get(key))
    return result

def mapCollectionFromJava (coll):
    """ Convert a Collection to a List. """
    result = []
    iter = coll.iterator();
    while iter.hasNext():
    return result

def mapFromJava (object):
    """ Convert a Java type to a Jython type. """
    if object is None: return object
    if   isinstance(object, util.Map):        
        result = mapMapFromJava(object)
    elif isinstance(object, util.Collection): 
        result = mapCollectionFromJava(object)
        result = object
    return result

def mapSeqToJava (seq):
    """ Convert a sequence to a Java ArrayList. """
    result = util.ArrayList(len(seq))
    for e in seq:
    return result

def mapDictToJava (dict):
    """ Convert a Dictionary to a Java HashMap. """
    result = util.HashMap()
    for key, value in dict.items():
        result.put(mapToJava(key), mapToJava(value))
    return result

def mapToJava (object):
    """ Convert a Jython type to a Java type. """
    if object is None: return object
    t = type(object)
    if   t == TupleType or t == ListType:  
        result = mapSeqToJava(object)
    elif t == DictType:  
        result = mapDictToJava(object)
        result = object
    return result

After using mapToJava, these types can be written to a After reading an object from a, you can use mapFromJava to convert the object back to a Jython type.

Note that these methods support a limited but broadly used set of built-in Jython types. Jython automatically converts value-like types such as numbers and strings. User defined classes are not supported.

Mapping Java types, continued

To continue the example, the following usage of the mapping functions discussed on the previous section as shown here:

data = (1,2,3, [1,2,3], [c for c in "Hello!"], "Hello!", {1:'one', 2:'two'})
print "data:", data
toJava = mapToJava(data)
print "toJava:", toJava
fromJava = mapFromJava(toJava)
print "fromJava:", fromJava


print "type(%s)=%s" % ("data", type(data))
print "type(%s)=%s" % ("toJava", type(toJava))
print "type(%s)=%s" % ("fromJava", type(fromJava))


data: (1, 2, 3, [1, 2, 3], ['H', 'e', 'l', 'l', 'o', '!'], 'Hello!', \
    {2: 'two', 1: 'one'})
toJava: [1, 2, 3, [1, 2, 3], [H, e, l, l, o, !], Hello!, {2=two, 1=one}]
fromJava: [1, 2, 3, [1, 2, 3], ['H', 'e', 'l', 'l', 'o', '!'], 'Hello!', \
    {2: 'two', 1: 'one'}]


Notice that the PyTuple became a PyJavaInstance and then a PyList. Also notice that the toJava form formats differently. This is because it is a Java object and it's being printed by the Java toString() method, not Jython repr() function. PyJavaInstance is a type Jython will pass as is to a Java API. Finally, notice that the data and fromJava values are the same except that the tuple is now an equivalent list. For more about Jython types see Appendix L: Jython types summary.

Parsing command lines

Frequently you need to extract command parameters with more processing than simple use of sys.argv provides. The parseArgs function can be used to get any command line arguments as a (tuple of) sequence of positional arguments and a dictionary of switches.

So, continuing the module excerpt (from Wrapping Java services in Jython and Mapping Java types, respectively), we see this:

def parseArgs (args, validNames, nameMap=None):
    """ Do some simple command line parsing. """
    # validNames is a dictionary of valid switch names -
    #   the value (if any) is a conversion function
    switches = {}
    positionals = []
    for arg in args:
        if arg[0] == '-':               # a switch
            text = arg[1:]
            name = text; value = None
            posn = text.find(':')       # any value comes after a :
            if posn >= 0:
                name = text[:posn]
                value = text[posn + 1:]
            if nameMap is not None:     # a map of valid switch names
                name = nameMap.get(name, name)
            if validNames.has_key(name):   # or - if name in validNames:
                mapper = validNames[name]
                if mapper is None: switches[name] = value
                else:              switches[name] = mapper(value)
                print "Unknown switch ignored -", name

        else:                           # a positional argument
    return positionals, switches

This function could be used as follows (in file

from sys import argv
from JavaUtils import parseArgs

switchDefs = {'s1':None, 's2':int, 's3':float, 's4':int}
args, switches = parseArgs(argv[1:], switchDefs)
print "args:", args
print "switches:", switches

For the command c:\>jython 1 2 3 -s1 -s2:1 ss -s4:2, it prints:

args: ['1', '2', '3', 'ss']
switches: {'s4': 2, 's2': 1, 's1': None}

Jython string processing

String operations and functions

Like most scripting languages, such as Perl and Rexx, Jython has extensive support for manipulating strings. This support is generally similar to the support provide by the Java language but it is often simpler and easier to use. In this section, we will talk about some of the more commonly used string operations and functions. See Part 1 of this tutorial and the Python Library Reference to learn more about string methods.

In the examples in the next few sections I will use the following values:

name ="Barry Feigenbaum"
addr = '12345 Any Street"
v1 = 100; v2 = v1 * 1.5; v3 = -v2; v4 = 1 / v2
s1 = "String 1"; s2 = "String 2"
sent = "The rain in Spain falls mainly on the plain."

Getting string forms of objects

To get a string representation of any value or expression (that is, object) use one of the following functions:

  • str(expr) creates a human-oriented string.
  • repr(expr) or `expr` creates (where possible) a computer-oriented string from which the eval function can re-create the value.

Note that for many types, including basic types, str(x) and repr(x) generate the same (or very similar) strings.

Basic string operations

A string is a built-in type, acting both as a value and as an object with methods. Strings support the basic operations of concatenation, indexing, containment, and formatting, as well as the other operations of immutable sequences. We'll go over the basic string operations, starting with concatenation.

We use the plus (+) operator to concatenate two strings. For example, the following line:

print "abc" + "xyz"

prints: abcxyz.

To select a character or characters (that is, a substring) from a string you use indexing. For example: "abcxwy"[2] yields c, while "abcxwy"[2:4] yields cx.

Many of the string functions test conditions, thus they are often used in conjunction with the if and while statements. Here's an example of how we could use containment testing to see if a character were contained in a string:

if ' ' in name: print "space found"

-- or --

if 'q' not in sent: print "q not found"

In addition to testing conditions, strings also support methods to test the nature of the string. These are islower, isupper, isalnum, isnum, isalpha, isspace, and istitle. These methods test to see if all the characters in the strings meet these conditions.

Additional methods

Strings support several methods that allow you to find and edit sub-strings, change case, and a host of other actions. To find a string in another string use the find/rfind or startswith/endswidth methods. For example:

if name.find(' ') >= 0: print "space found"

-- or --

if name.find("Jones") < 0: print "Jones not in name"

Sometimes you need to edit the content of a string, for example to change its case or insert or remove text from it. Jython supplies several methods to do this. To change case, Jython has the lower, upper, swapcase, title, and capitalize methods. To change the text of a string, use the replace method. For example, to match strings often you want to ignore case or you may want to replace sub-strings:

if  s1.lower() == s2.lower(): print "equal"

-- or --

newaddr = addr.replace("Street", "St.")

Often strings have extra blanks around them that are not important, such as when the string is entered by a user. To remove these extra blanks use the lstrip, rstrip, or strip methods. For example, to match a command entered by a user:

cmd = raw_input("Enter a command")
if cmd.lstrip.startswith("run "): 
    print "run command found"

Often you need to break strings into parts, such as the words in a sentence or join multiple strings into one string. Jython supports the split, splitlines, and join functions to do this. The split method splits a line into words, while splitlines splits a file of lines into separate lines. The join method reverses split. You can also join strings by concatenation as discussed above. For example, to extract the words from a sentence and then rebuild the sentence use:

words = sent.split(' ')   # use space to separate words
sent2 = ' '.join(words)   # use space between words

Formatting program variables

It is very easy to format local or global variables using the modulus (%) operator. The locals and globals functions return dictionaries for all the local and global (respectively) variables. For example:

fname = "Barry"; lname = "Feigenbaum"
address = "1234 any St."
city = "Anytown"; state = "TX"; zip = "12345"
age = 30
children = 3
print "Hello %(fname)s from %(city)s, %(state)s." % locals()

prints Hello Barry from Anytown, TX.

See Appendix J: Formatting strings and values for more about formatting program variables.

Format operator examples

Below are some format (%) operator examples. See Appendix J: Formatting strings and values for more examples.

"Hello %s" % "Barry"Hello Barry
"Count: %i, " "Avg Cost: $%.2f; " "Max Cost: $%.2f" % (10, 10.5, 50.25)Count: 10, Avg Cost: $10.50; Max Cost: $50.25
"This is %i%%" % 10This is 10%
"My name is %(first)s %(last)s!" % {'last':'Feigenbaum', 'first':'Barry', 'mi':'A'}My name is Barry Feigenbaum!

Using C-style printf

For those familiar with C's printf("... %x ...", v1, ..., vN) function, a similar but enhanced service can be added in Jython, as shown here:

def printf(stream, format, *pargs, **kwargs):

                        # see Printing to files for more information
    if   pargs:
        print >>stream, format % pargs
    elif kwargs:
        print >>stream, format % kwargs
        print >>stream, format


Using the above printf function definition, the following examples:

from sys import stdout

printf(stdout, "%s is %.1f years old and has %i children", 
       fname, age, children)

printf(stdout, "The %(name)s building has %(floors)d floors", 
       floors=105, name="Empire State")

printf(stdout, "Hello World!")


Barry is 30.0 years old and has 3 children
The Empire State building has 105 floors
Hello World!

Pretty printing

You can use the pprint module functions, in particular the pformat function, to print complex data structures in a formatted form. For example, this code:

data = [[1,2,3], [4,5,6],{'1':'one', '2':'two'}, 
        "jsdlkjdlkadlkad", [i for i in xrange(10)]]
print "Unformatted:"; print data


from pprint import pformat
print "Formatted:"; print pformat(data)

prints the following:

[[1, 2, 3], [4, 5, 6], {'2': 'two', '1': 'one'}, \
    'jsdlkjdlkadlkad', [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]]

[[1, 2, 3],
 [4, 5, 6],
 {'2': 'two', '1': 'one'},
 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]]

Using string functions

As an example of using the string operations from String operations and functions, the program (listed below) takes paragraphs of input and formats them into pages. The text may be left-, center-, right-aligned, or justified. Page margins may be specified. Header and/or footer text may be supplied.

See Related topics for some sample results of using this program.

import sys

def stripLines (lines):
    """ Removed extra whitespace (that is, newlines). """
    newlines = []
    for line in lines:
        line = line.strip()
    return newlines

def splitParagraphs (lines):
    """ Splits a set of lines into paragraphs.  """
    paras = []
    para = ""
    for line in lines:
        if len(line) > 0:       # in paragraph
            para += ' ' + line
        else:                   # between paragraphs
            para = para.strip()
            if len(para) > 0:
                para = ""
    return paras

class Formatter:
    """ Formats and prints paragraphs.  """

    def __init__ (self, stream, pagelen=66, linewidth=85, 
                        lmargin=10, rmargin=10, pindent=5,
                        headers=None, footers=None): = stream        # stream to print on

        # format settings
        self.pagelen = pagelen 
        self.pindent = pindent 
        self.linewidth = linewidth  
        self.lmargin = lmargin    
        self.rmargin = rmargin    
        self.headers = headers    
        self.footers = footers    
        self.alignment = alignment

        self.pagecount = 1         # current page
        self.linecount = 0         # current line
    def genLine (self, line):
        print >>, line
        self.linecount += 1

    def outputLine (self, line):
        if not (self.linecount == 0 and len(line) == 0):
    def newPage (self):
        if self.headers:
    def padPage (self):
        while self.linecount < self.pagelen:
    def endPage (self):
        if self.footers:
            if len(self.footers) + self.linecount < self.pagelen:
            if self.linecount < self.pagelen:
        self.linecount = 0
        self.pagecount += 1
        self.genLine('-' * 20)
    def testEndPage (self):
        if self.footers:
            if len(self.footers) + 1 + self.linecount >= self.pagelen:
            if self.linecount >= self.pagelen:

    def padLine (self, line, firstline=0, lastline=0):
        """ Add spaces as needed by alignment mode.  """

        if   self.alignment == "left":
            adjust = firstline * self.pindent
            #line = line

        elif self.alignment == "center":
            adjust = 0
            pad = self.linewidth - adjust - len(line)
            line = ' ' * (pad / 2) + line

        elif self.alignment == "right":
            adjust = 0
            pad = self.linewidth - adjust - len(line)
            line = ' ' * pad + line

        elif self.alignment == "justify":
            adjust = firstline * self.pindent
            pad = self.linewidth - adjust - len(line)
            line = ""

            # add 1+ spaces between words to extend line
            words = line.split()
            xpad = pad                       
            for word in words:
                line += word + ' '
                if not lastline and xpad > 0:
                    line += ' '  * (pad / len(words) + 1)
                    xpad -= 1
            line = line.strip()

        return ' ' * adjust + line 

    def format (self, line, firstline=0, lastline=0):
        # indent by left margin
        return ' ' * self.lmargin + \
               self.padLine(line.strip(), firstline, lastline)

    def formatParagraph (self, para):
        lcount = 0
        adjust = self.pindent
        line = ""

        # process by words
        words = para.split(' ')
        for word in words:
            line += ' '
            # about to get too long
            if len(line) + len(word) > self.linewidth - adjust:
                line = self.format(line, lcount == 0, 0)
                line = "" 
                lcount += 1
                adjust = 0
            line += word
        # output last (only) line
        if len(line) > 0:
            line = self.format(line, lcount == 0, 1)

    def outputHeader (self):
        for line in self.headers:
            self.genLine(' ' * self.lmargin +
    def outputFooter (self):
        for line in self.footers:
            self.genLine(' ' * self.lmargin +
    def outputPages (self, paras):
        """ Format and print the paragraphs. """
        for para in paras:

Processing regular expressions

About regular expressions

As an extension to the find and replace functions described in String operations and functions, Jython supports regular expressions. Regular expressions (RE) are strings that contain plain match text and control characters and provide an extremely powerful string search and replace facility. Jython supports (at least) the following forms of regular expressions:

  • re module is a built-in part of Jython.
  • Java works if you're running Jython on Java 1.4 or above.
  • Apache ORO works if you add the ORO package to your CLASSPATH.

Regular expression formats

The simplest RE is an exact string to match. More complex REs include special control characters. The control characters allow you to create patterns of matching strings. For more information on RE syntax and options see Appendix H: Regular expression control characters and the Python Library Reference.

[Jennette, from Barry: We need to get the spacing right in this table, I have multiple nbsp's that show as only one space.]-->

Below are some example REs and the strings they match:

Control characterRegular expressionMatchesDoes not match
-- none --abcabcab
. - any charactera.cabc
a c
* - optional repeating subpatterna.*cabc
a c
? - optional subpatterna.?cabcac
+ - required repeating subpatterna.+cabc
...|... - choice of subpatternabc|defabcef
(...) - groupinga(xx)|(yy)caxxc
(...)* - repeating groupinga(xx)*cac
(...)+ - required repeating groupinga(xx)+caxxc
\c - match a special character\.\?\*\+.?*+?.*+
\s - matches white spacea\s*zaz
a z
a    z
z a

Regular expressions functions

The Jython re module provides support for regular expressions. re's primary functions are findall, match, and search to find strings, and sub and subn to edit them. The match function looks at the start of a string, the search function looks anywhere in a string, and the findall function repeats search for each possible match in the string. search is (by far) the most used of the regular expression functions.

Here are some of the most common RE functions:

match(pattern, string {, options})Matches pattern at the string start
search(pattern, string {, options})Matches pattern somewhere in the string
findall(pattern, string)Matches all occurrences of pattern in the string
split(pattern, string {, max})Splits the string at matching points and returns the results in a list
sub(pattern, repl, string {, max})Substitutes the match with repl for max or all occurrences; returns the result
subn(pattern, repl, string {, max})Substitutes the match with repl for max or all occurrences; returns the tuple (result, count)

Note that the matching functions return None if no match is found. Otherwise the match functions will return a Match object from which details of the match can be found. See the Python Library Reference for more information on Match objects.

Two function examples

Let's take a look at some examples of regular expressions functions in action:

import re

# do a fancy string match
if"^\s*barry\s+feigenbaum\s*$", name, re.I):
   print "It's Barry alright"

# replace the first name with an initial
name2 = re.sub(r"(B|b)arry", "B.", name)

If you are going to use the same pattern repeatedly, such as in a loop, you can speed up execution by using the compile function to compile the regular expression into a Pattern object and then using that object's methods, as shown here:

import re
patstr = r"\s*abc\s*"
pat = re.compile(patstr)
# print all lines matching patstr
for s in stringList:
    if pat.match(s, re.I): print "%r matches %r" % (s, patstr)

Regular expression example: Grep

The following simplified version of the Grep utility (from offers a more complete example of a Jython string function.

""" A simplified form of Grep. """

import sys, re

if len(sys.argv) != 3:
    print "Usage: jython <pattern> <file>"
    # process the arguments
    pgm, patstr, filestr = sys.argv
    print "Grep - pattern: %r file: %s" % (patstr, filestr)
    pat = re.compile(patstr)  # prepare the pattern

                        # see File I/O in Jython
                     for more information
    file = open(filestr)      # access file for read
    lines = file.readlines()  # get the file

    count = 0
    # process each line
    for line in lines:
        match =    # try a match
        if match:                   # got a match
            print line
            print "Matching groups: " + str(match.groups())
            count += 1
    print "%i match(es)" % count

When run on the words.txt file from File I/O in Jython , the program produces the following result:

C:\Articles>jython "(\w*)!" words.txt
Grep - pattern: '(\\w*)!' file: words.txt
How many times must I say it; Again! again! and again!

Matched on: ('Again',)
Singing in the rain! I'm singing in the rain! \
    Just singing, just singing, in the rain!

Matched on: ('rain',)
2 match(es)

File I/O in Jython

Using files

In addition to the Java platform's file-related APIs (packages and, in Java 1.4, java.nio), Jython provides simple yet powerful access to files using the File type and services in the os, os.path, and sys modules. (See Appendix F: The os module , Appendix G: The os.path module , Appendix E: The sys module and the Python Reference Manual for more details on the os and os.path packages.)

We'll start with a look at some basic file-access operations. A File object is created using the built-in open function, shown below, where path is the path to the file, mode is the access mode string, and size is the suggested buffer size:

file = open(path {, mode {, size}})

The mode string has the following syntax: (r|w|a){+}{b}; the default mode is r. Here is a listing of all the available access mode strings:

  • r: read
  • w: write
  • a: append to the end of the file
  • +: update
  • b: binary (vs. text)

The name of the file is accessed through the name attribute. The mode of the file is accessed through the mode attribute.

File access methods

Files support the following methods:

close()Flush and close an open file
flush()Outputs any buffered data
read({size})Reads up to size (or the whole file)
readline({size})Read a line (including ending '\n') up to size
readlines()Reads the file and returns a list of lines (including ending '\n')
seek(offset {, mode})Seek to a position, mode: 0 - start of file, 1 - current offset, 2 - end of file
tell()Return the current offset
truncate({size})Truncate (delete extra content) to current offset or specified size
write(string)Write the string to a file. To write lines, end the string in '\n'
writelines(lines)Write the list as a set of strings. To write lines, end each string in '\n'

Simple file processing examples

We'll look at a couple of simple file processing examples, starting with the file copy program below:

import sys

f = open(sys.argv[1], "rb")    # open binary for reading
bin =
f = open(sys.argv[2], "wb")    # open binary (truncated) for write

And here is a text file sort procedure:

import sys

f = open(sys.argv[1], "r")    # read the file by lines
lines = f.readlines()
lines.sort()                  # sort and print the lines
print "File %s sorted" %
print lines

A word-counting program in Jython

As a more complete example of file processing, study the following word-counting program:

import sys

def clean (word):
    """ Remove any punctuation and map to a common case. """
    word = word.lower()
    # remove any special characters
    while word and word[-1] in ".,;!": word = word[:-1]
    while word and word[0] in ".,;!": word = word[1:]
    return word

words = {}  # set of unique words and counts

if len(sys.argv) != 2:
    print "Usage: jython <file>"
    file = open(sys.argv[1])  # access file for read
    lines = file.readlines()  # get the file

    # process each line
    for line in lines:
        # process each word in the line
        for word in line.split():
            word = clean(word)
            words[word] = words.get(word, 0) + 1   # update the count

    # report the results
    keys = words.keys()
    for word in keys:
        print "%-5i %s" % (words[word], word)

Output of words.txt

Given the following input file (words.txt)

Now is the time for all good men to come to the aid of their country.
The rain in Spain falls mainly on the plain.
How many times must I say it; Again! again! and again!
Singing in the rain! I'm singing in the rain! \
    Just singing, just singing, in the rain!

the word-counting program (from A word-counting program in Jython) would return the following results (wrapped into two columns to save space):

3     again             1     many       
1     aid               1     men        
1     all               1     must       
1     and               1     now        
1     come              1     of         
1     country           1     on         
1     falls             1     plain      
1     for               4     rain       
1     good              1     say        
1     how               4     singing    
1     i                 1     spain      
1     i'm               7     the        
4     in                1     their      
1     is                1     time       
1     it                1     times      
2     just              2     to         
1     mainly

The word-counting program in Java code

Let's take a look at the word-counting script re-implemented in the Java language. Notice the extensive use of types in declarations and type-casts in the assignment statements. As you can see, the Java code is significantly larger (approximately two times) and arguably far more cryptic.

import java.util.*;
import java.text.*;

public class WordCounter
    protected static final String specials = ".,;!";

    /** Remove any punctuation and map to a common case. */
    protected static String clean(String word) {
        word = word.toLowerCase();
        // remove any special characters
        while (word.length() > 0 && 
               specials.indexOf(word.charAt(word.length() - 1)) >= 0) {
            word = word.substring(0, word.length() - 1);

        while (word.length() > 0 && 
               specials.indexOf(word.charAt(0)) >= 0) {
            word = word.substring(1);
        return word;

    protected static Map words = new HashMap();

    public static void main(String[] args) throws IOException {
        if (args.length != 1) {
            System.out.println("Usage: java WordCounter <file>");
        else {                               
            // access file for read
            FileInputStream fis = new FileInputStream(args[0]);
            DataInputStream dis = new DataInputStream(fis);
            List lines = new ArrayList();
            // get the file
            for (String line = dis.readLine(); 
                 line != null; 
                 line = dis.readLine()) {

            // process each line
            for (int i = 0; i < lines.size(); i++) {
                String line = (String)lines.get(i);
                System.out.println("Processing: " + line);
                String[] xwords = line.split("\\s+");
                for (int w = 0; w < xwords.length; w++) {
                    String word = clean(xwords[w]);
                    if (word.length() > 0) {
                        Integer count = (Integer)words.get(word);
                        if (count == null) {
                            count = new Integer(0);
                        // update the count
                                  new Integer(count.intValue() + 1));  

            // report the results
            String[] keys = (String[])words.keySet().
                                 toArray(new String[words.size()]);

            MessageFormat form = new MessageFormat(
                            "{0,number, #########0} {1}");
            for (int i = 0; i < keys.length; i++) {
                          new Object[] {words.get(keys[i]), keys[i]}));

Printing to files

The print statement can print to a file by use of the ">>" operator. By default it prints to the console (actually the value of sys.stdout). For example, the following commands are equivalent:

print "Hello World!"
import sys
print >>sys.stdout, "Hello world!"

Jython allows alternate target files. For example, to print to the standard error stream use:

print >>sys.stderr, "Hello world!"

To print to a file use:

f = open("myfile", "w")
for i in range(10):
    print >>f, "Line", i

And to add to the end of a file use:

f = open("myfile", "a")
print >>f, "Added line"

Saving objects persistently

Sometimes you may want to save an object persistently (beyond the lifetime of the program that creates it) or send it to another application. To do this you need to serialize (or pickle) the object so it can be placed in a file or on a stream. You then need to de-serialize (or un-pickle) the object to use it again. Jython provides a module, pickle, that makes this very easy. The pickle module contains the following useful functions:

load(file)Returns an object re-created from a previously created image in a file.
loads(string)Returns an object recreated from a previously created image in a string.
dump(object, file {, bin})Stores an object image into a file. If bin is omitted or false, use a text representation; else a binary representation (which is typically smaller).
dumps(object{, bin})Returns a string containing the image of the object. If bin is omitted or false, use a text representation; else a binary representation (which is typically smaller).

A pickling example

Here's an example of pickle at work. The following code sequence

import pickle

class Data:
    def __init__ (self, x, y):
        self.__x = x
        self.__y = y

    def __str__ (self):
        return "Data(%s,%s)" % (self.__x, self.__y)

    def __eq__ (self, other):
        return self.__x == other.__x and self.__y == other.__y

data = Data(10, "hello")

file = open("data.pic", 'w')
pickle.dump(data, file)

file = open("data.pic", 'r')
newdata = pickle.load(file)

print "data:", data
print "newdata:", newdata
print "data is newdata:", data is newdata
print "data == newdata:", data == newdata

prints this:

data: Data(10,hello)
newdata: Data(10,hello)
data is newdata: 0 (false)
data == newdata: 1 (true)

The file created is in (semi-)readable plain text. For example, the above code created the file data.pic:


Note that Jython cannot pickle objects that are Java objects, reference Java objects, or subclass Java classes. To do this you need to use the and classes.

Object shelves

As shown in the previous section, Jython can store objects into a file. Using a file per object can cause problems (that is, it can waste space and you will need to name each file). Jython supports a file that can hold multiple objects, called a shelf. A shelf acts much like a persistent dictionary. To create shelves, use the open function of module shelve. For example, the following code:

import shelve, sys

def printshelf (shelf, stream=sys.stdout):  # print the entries in a shelf
    for k in shelf.keys():
        print >>stream, k, '=', shelf[k]

def clearshelf (shelf):                     # remove all keys in the shelf
    for k in shelf.keys():
        del shelf[k]

# create shelf
shelf ="test.shelf")
shelf["x"] = [1,2,3,4]
shelf["y"] = {'a':1, 'b':2, 'c':3}

# update shelf
shelf ="test.shelf")
shelf["z"] = sys.argv[1]

# verify shelf persistent
shelf ="test.shelf")

produces this output (with argument "This is a test string"):

x = [1, 2, 3, 4]
y = {'b': 2, 'a': 1, 'c': 3}

x = [1, 2, 3, 4]
y = {'b': 2, 'a': 1, 'c': 3}

x = [1, 2, 3, 4]
z = This is a test string
y = {'b': 2, 'a': 1, 'c': 3}

x = [1, 2, 3, 4]
z = This is a test string
y = {'b': 2, 'a': 1, 'c': 3}

Note that the open function produces two files based on the file name passed to open:

  • <filename>.dir is a directory into the persistent data
  • <filename>.dat is the saved persistent object data

A simple Swing GUI

The Factorial Calculator

We'll close this second installment of the "Introduction to Jython" tutorial with a complete program that encompasses many of the details we have so far discussed. The Factorial Calculator is a GUI application written entirely in Jython. It calculates the value of x! (x factorial) for any positive integer value. Because x! can be very large, this example takes advantage of Jython's ability to process very large integers. Calculations for large values of x (say, > 10000) can take several minutes, so the user interface includes a progress bar and a Cancel button to interrupt a calculation.

In the sections that follow, you can see the two most essential components of the Factorial Calculator: the class that supplies the factorial calculation engine, and the set of classes that comprise the GUI. The complete, runnable code for the Factorial Calculator is available for download in Related topics. Note that in order to completely understand the GUI code you should have some experience with creating Swing GUIs. Even without this prior knowledge, you should be able to discern many elements of the code from our prior discussion throughout this tutorial.

To get started, let's see what our GUI application looks like. Here's a screenshot of the GUI showing the result of calculating 100! (that is, 100 factorial).

The factorial engine:

Factorial is the class that supplies the factorial calculation engine. It consists of a sequence of code with additional explanation lines (identified by --) added.

-- import the needed modules
import sys
import exceptions

-- create the Factorial class, a general purpose factorial calculation engine
class Factorial:
    """A general purpose factorial calculation engine"""

-- define the constructor
    def __init__ (self):
        self.__listeners = []
        self.__cancelled = 0

-- allow other classes to register event listeners; 
---    used to track calculation progress
-- A "listener" is a function that takes an integer % argument
    def addListener (self, listener):
        if listener not in self.__listeners:

    def addListeners (self, listeners):
        for l in listeners:

    def removeListener (self, listener):

    def removeListeners (self, listeners):
        for l in listeners:

    def fireListeners (self, value):  # notify all listeners
        for func in self.__listeners:

-- allow others to cancel a long running calculation
    def cancel (self):
        self.__cancelled = 1

-- perform the factorial calculation; 
--     may take a long time (many minutes) for big numbers
    def calculate (self, value):
        if type(value) != type(0) or value < 0:
            raise ValueError,  \
               "only positive integers supported: " + str(value)

        self.__cancelled = 0
        result = 1L
        self.fireListeners(0)   # 0% done
        # calculate factorial -- may take quite a while
        if value > 1:           # need to do calculation
            last = 0
            # using iteration (vs. recursion) to increase performance
            # and eliminate any stack overflow possibility
            for x in xrange(1, value + 1):
                if self.__cancelled: break  # early abort requested
                result = result * x         # calc next value
                next = x * 100 / value
                if next != last:            # signal progress 
                    last = next
        self.fireListeners(100)  # 100% done
        if self.__cancelled: result = -1
        return result

# test case
if __name__ == "__main__":
    print sys.argv[0], "running..."
    fac = Factorial()

    def doFac (value):
            print "For", value, "result =", fac.calculate(value)
        except ValueError, e: 
            print "Exception -", e


The GUI:

Here you can see the set of classes that supplies the factorial GUI. The set consists of a sequence of code with additional explanation lines (identified by --) added.

-- import the needed modules
import sys
import string
from types import *

from java import lang
from java import awt
from java.awt import event as awtevent
from javax import swing

from factor import Factorial

-- PromptedValueLayout is a customized Java LayoutManager not discussed here 
--    but included with the resources
from import PromptedValueLayout as ValueLayout

-- support asynchronous processing
class LongRunningTask(lang.Thread):
    def __init__ (self, runner, param=None):
        self.__runner = runner   # function to run
        self.__param = param     # function parameter (if any)
        self.complete = 0
        self.running = 0

-- Java thread body
    def run (self):
        self.complete = 0; self.running = 1
        if self.__param is not None: 
            self.result = self.__runner(self.__param)
            self.result = self.__runner()
        self.complete = 1; self.running = 0

-- start a long running activity
def doAsync (func, param):
    LongRunningTask(func, param).start()

-- Swing GUI services must be called only on the AWT event thread,
class SwingNotifier(lang.Runnable):
    def __init__ (self, processor, param=None):
        self.__runner = processor  # function to do GUI updates
        self.__param = param       # function parameter (if any)

-- Java thread body
    def run (self):
        if self.__param is not None: self.__runner(self.__param)
        else:                        self.__runner()

    def execute (self):
     -- define and construct a GUI for factorial calculation
class FactorialGui(swing.JPanel):
    """Create and process the GUI."""

    def __init__ (self, engine):
        self.__engine = engine

    def update (self, value):          # do on AWT thread
        SwingNotifier(self.updateProgress, value).execute()

    def updateProgress (self, value):  # display progress updates
        self.__progressBar.value = value

-- Calculate button press handler
    def doCalc (self, event):          # request a factorial
        self.__outputArea.text = ""
        ivalue = self.__inputField.text # get value to calculate
        value = -1
        try: value = int(ivalue)        # convert it
        except: pass
        if value < 0:                   # verify it
           self.__statusLabel.text = \
               "Cannot make into a positive integer value: " + ivalue
            self.__calcButton.enabled = 0
            self.__cancelButton.enabled = 1
            msg = "Calculating factorial of %i..." % value
            if value > 25000: msg += \ 
                 "; May take a very long time to complete!"
            self.__statusLabel.text = msg  # tell user we're busy
            doAsync(self.calcFac, value)   # do the calculation

-- main calculation worker
    def calcFac (self, value): 
        stime = lang.System.currentTimeMillis()
        fac = self.__engine.calculate(value)   # time calculation
        etime = lang.System.currentTimeMillis()
        svalue = ""; order = 0
        if fac >= 0:          # we have a result, not cancelled
            svalue = str(fac); order = len(svalue) - 1
            formatted = ""
            while len(svalue) > 100:  # wrap long numbers
                formatted += svalue[0:100] + '\n'
                svalue = svalue[100:]
            formatted += svalue
            svalue = formatted
        ftime = lang.System.currentTimeMillis()

        SwingNotifier(self.setResult, \
          (svalue, order, ftime - stime, etime - stime)).execute()

-- display the result
    def setResult (self, values):
        svalue, order, ttime, ftime = values
        self.__cancelButton.enabled = 0
        if len(svalue) > 0:
            self.__statusLabel.text = \
               "Setting result - Order: 10**%i" % order
            self.__outputArea.text = svalue
            self.__statusLabel.text = \
                "Total time: %ims, Calc time: %ims, Order: 10**%i" % \
                      (ttime, ftime, order)
            self.__statusLabel.text = "Cancelled"

        self.__calcButton.enabled = 1

-- Cancel button press handler
    def doCancel (self, event):       # request a cancel
        self.__cancelButton.enabled = 0

-- create (layout) the GUI
    def createGui (self):             # build the GUI
        self.layout = awt.BorderLayout()

        progB = self.__progressBar = \
            swing.JProgressBar(0, 100, stringPainted=1);

        inf = self.__inputField = swing.JTextField(5)
        inl = swing.JLabel("Calculate value of:", swing.JLabel.RIGHT)
        inl.labelFor = inf

        outf = self.__outputArea = swing.JTextArea()
        outl = swing.JLabel("Result:", swing.JLabel.RIGHT)
        outl.labelFor = outf

        calcb = self.__calcButton = \
            swing.JButton("Calculate", actionPerformed=self.doCalc,
                          enabled=1, mnemonic=awtevent.KeyEvent.VK_C)
        cancelb = self.__cancelButton = \
             swing.JButton("Cancel", actionPerformed=self.doCancel,
                          enabled=0, mnemonic=awtevent.KeyEvent.VK_L)

        vl = ValueLayout(5, 5)
        inp = swing.JPanel(vl)
        vl.setLayoutAlignmentX(inp, 0.2)
        inp.add(inl); inp.add(inf, inl)
        self.add(inp, awt.BorderLayout.NORTH)

        vl = ValueLayout(5, 5)
        outp = swing.JPanel(vl)
        vl.setLayoutAlignmentX(outp, 0.2)
        outp.add(outl); outp.add(swing.JScrollPane(outf), outl)

        xoutp = swing.JPanel(awt.BorderLayout())
        xoutp.add(progB, awt.BorderLayout.NORTH)
        xoutp.add(outp, awt.BorderLayout.CENTER)

        self.add(xoutp, awt.BorderLayout.CENTER)

        sp = swing.JPanel(awt.BorderLayout())

        bp = swing.JPanel()
        sp.add(bp, awt.BorderLayout.NORTH)

        sl = self.__statusLabel = swing.JLabel(" ")
        sp.add(sl, awt.BorderLayout.SOUTH)
        self.add(sp, awt.BorderLayout.SOUTH)

-- main entry point; launches the GUI in a frame
if __name__ == "__main__":
    print sys.argv[0], "running..."
    frame = swing.JFrame("Factorial Calculator",
    cp = frame.contentPane
    cp.layout = awt.BorderLayout()
    cp.add( FactorialGui(Factorial()) )
    frame.size = 900, 500
    frame.visible = 1



This completes the two-part "Introduction to Jython" tutorial. While much of the tutorial functions as an overview, I hope I have provided you with enough advanced discussion, code examples, and incentive to proceed into more hands-on learning, specifically by developing your own programs in Jython.

In my opinion, Jython does for the Java platform what Visual Basic does for Microsoft's .NET: It provides much easier access to a complex development and execution environment. While easy to use, Jython improves upon the Java language by incorporating features the Java language lacks (some of which are also available today in .NET languages such as C#) without sacrificing any of the Java platform's capability (unless you count compile-time-type checking or a small reduction in effective performance).

We've discussed many of Jython's enhancements in this tutorial -- including for each iteration, property methods accessible as attributes, collection literals, generic collections that hold basic types (such as integers), generic functions, first-class functions, overloadable operators, C-like printf formatting, functions as event handlers, and dynamic code execution. Some of these features are so compelling that they will be included in the next version of the Java platform (that is, 1.5). Of course, with Jython you don't have to wait -- you can begin using them today!


Appendix A: Character codes for array types

The table below lists the character codes for Jython array types (see Java arrays from Jython).

Character type code Corresponding Java type
'z' Boolean
'c' char
'b' byte
'h' short
'i' int
'l' long
'f' float
'd' double

Note: The above table is from

Appendix B: Common overloaded operators and methods

The following are the operators that are commonly (additional operators can be) overloaded:

Operator Function to override Comment(s)
x + y
x += y
__add__(self, other)
__radd__ (self, other)
__iadd__(self, other)
__pos__ self)
Implements + operator
x - y
x -= y
__sub__(self, other)
__rsub__(self, other)
__isub__(self, other)
Implements - operator
x * y
x *= y
__mul__(self, other)
__rmul__(self, other)
__imul__(self, other)
Implements * operator
x / y
x /= y
__div__(self, other)
__rdiv__(self, other)
__idiv__(self, other)
Implements / operator
x % y
x %= y
__mod__(self, other)
__rmod__(self, other)
__imod__(self, other)
Implements % operator
x & y
x &= y
__and__(self, other)
__rand__(self, other)
__iand__(self, other)
Implements & operator
x | y
x |= y
__or__(self, other)
__ror__(self, other)
__ior__(self, other)
Implements | operator
x ^ y
x ^= y
__xor__(self, other)
__rxor__(self, other)
__ixor__(self, other)
Implements ^ operator
~ x __invert__(self) Implements ~ operator
x << y
x <<= y
__lshift__(self, other)
__rlshift__(self, other)
__ilshift__(self, other)
Implements << operator
x >> y
x >>= y
__rshift__(self, other)
__ rrshift__(self, other)
__ irshift__(self, other)
Implements >> operator
x ** y
x **= y
__pow__(self, other)
__rpow__(self, other)
__ipow__(self, other)
Implements ** operator
divmod(x,y) __divmod__(self, other)
__rdivmod__(self, other)
Implements divmod()
x < y __lt__(self, other) Implements < operator. This should return the opposite value returned by __ge__.
x <= y __le__(self, other) Implements <= operator. This should return the opposite value returned by __gt__.
x > y __gt__(self, other) Implements > operator. This should return the opposite value returned by __le__.
x >= y __ge__(self, other) Implements >= operator. This should return the opposite value returned by __lt__.
x == y __eq__(self, other) Implements == operator. This should return the opposite value returned by __ne__.
x != y
x <> y
__ne__(self, other) Implements != operator. This should return the opposite value returned by __eq__.
cmp(x,y) __cmp__(self, other) Implements cmp(); also used for relational tests if above specific overrides are not defined. This should return a < 0, 0 or > 0 value (say -1, 0 or 1).
x __nonzero__(self) Implements logical tests. This should return 0 or 1.
hash(x) __hash__(self) Implements hash(). Returns an integer value. Instances that are equal (that is, __eq__ returns true) should return the same __hash__ value (that is, (x == y) and (hash(x) == hash(y)) should be true. Similar to java.lang.Object.hashCode().
abs(x) __abs__(self) Implements abs()
int(x) __int__(self) Implements int()
long(x) __long__(self) Implements long()
float(x) __float__(self) Implements float()
complex(x) __complex__(self) Implements complex()
oct(x) __oct__(self) Implements oct()
hex(x) __hex__(self) Implements hex()
coerce(x,y) __coerce__(self, other) Implements coerce()
y = __getattr__ (self, name) Implements attribute lookup = y __setattr__ (self, name) Implements attribute creation/update
del __delattr__ (self, name) Implements attribute removal
y = c[i] __getitem_ (self, i) Implements item lookup
c[i] = y __setitem__ (self, i) Implements item creation/update
del c[i] __delitem__ (self, i) Implements item removal
x(arg, ...) __call__ (self, arg, ...) Implements the call operator
len(c) __len__ (self) Implements len()
x in c
x not in c
__contains__ (self, other) Implements in operator
class() __init__ (self, ...) Instance constructor; called when the class is created
del x __del__ (self) Instance destructor; called just before being deallocated
-- or --
__repr__(self) Implements repr() on this class
str(x) __str__(self) Implements str() on this class; Jython uses __repr__ if __str__ is not defined. str() is used like x.toString() in Java

Note: For the binary operators, the __xxx__ form is used when the left (or both) argument implements the function; the __rxxx__ form is used only if the right argument implements the function and the left argument does not; the __ixxx__ form is used to implement the augmented assignment (x ?= y) operation. See the Python Reference Manual for more details and overload-able functions.

Appendix C: Jython debugger commands

The debugger provides the following functions/features:

Command Arguments Function
h, help -- none -- List the available commands
w, where -- none -- Shows the current stack trace
d, down -- none -- Move down one stack frame
u, up -- none -- Move up one stack frame
b, break line# | function, condition_expr Set a breakpoint at a line number or function with an optional expression to evaluate - stop only if true
tbreak line# | function, condition_expr Set a breakpoint at a line number or function with an optional expression to evaluate - stop only if true; the breakpoint is automatically cleared when hit
cl, clear bpid... Clears all or listed breakpoints
enable bpid... Enables breakpoints
disable bpid... Disabled breakpoints
ignore bpid, count Sets the breakpoint ignore (auto-resume) count
condition bpid, condition_expr Sets the breakpoint condition expression
s, step -- none -- Steps over the next line, possibly into a function
n, next -- none -- Resume until the next line is reached
r, return -- none -- Resume until the current function returns
c, cont, continue -- none -- Resume execution
j, jump line# Set a new current line
l, list line#1, line#1 Lists source from line#1 to line#2, if omitted, then list the lines around the current line
a, args -- none -- Show the arguments of the current function
p, pp expr Evaluate the expression and print its result; pp formats the result
print expr Do the print statement, that is, - !print expr
alias name, expr Create a named expression to simplify printing of repeated values
unalias name Delete an alias
q, quit -- none -- End the debugging session
! statement Execute the Jython statement

Note: entering a blank line repeats the prior command.

Appendix D: Jython to/from Java type mapping

Jython uses these rules to map parameter types:

Java Parameter Types Allowed Python Types
char String (must have length 1)
Boolean Integer (true = nonzero)
byte, short, int, long Integer
float, double Float
java.lang.String, byte[], char[] String
java.lang.Class Class or JavaClass
Foobar[] Array (must contain objects of class or subclass of Foobar)
java.lang.Object String->java.lang.String, all others unchanged
org.python.core.PyObject All unchanged
Foobar Instance --> Foobar (if Instance is subclass of Foobar); JavaInstance --> Foobar (if JavaInstance is instance of Foobar or subclass)

Jython uses these rules to map return value types:

Java Return Type Returned Python Type
char String (of length 1)
Boolean Integer (true = 1, false = 0)
byte, short, int, long Integer
float, double Float
java.lang.String String
java.lang.Class JavaClass which represents given Java class
Foobar[] Array (containing objects of class or subclass of Foobar)
org.python.core.PyObject (or subclass) Unchanged
Foobar JavaInstance which represents the Java Class Foobar

Note: the above two tables are from the site.

Appendix E: The sys module

The sys module has some important variables:

Variable Comment(s)
argv The arguments supplied to the main module. argv[0] is the program name, argv[1] is the first argument and so on
Largest/smallest integer value
platform The version of Java Jython is running on
path The module search path
Standard input, output and error streams
modules List of currently loaded modules
Jython version and details

The sys module has some important functions:

Function Comment(s)
exit(int) Exits the program
exc_info() Get information on the most recent exception

See the Python Library Reference for more information.

Appendix F: The os module

The os module has some important variables:

Variable Comment(s)
name Type of host
curdir String to represent the current directory
pardir String to represent the parent directory
sep String to separate directories in a path
pathsep String to separate paths in a path set string
linesep String to separate text lines
environ The current environment string

The sys module has some important functions:

Function Comment(s)
getcwd() Get the current directory
Create/delete a directory
-- or --
Delete a file
listdir(path) List the files in a path
rename(path, new) Renames a file/directory to new
system(command) Run a shell command

See the Python Library Reference for more information.

Appendix G: The os.path module

The os.path module has some important functions:

Function Comment(s)
exists(path) True is path exists
Returns the absolute form of the path
Returns the normalized form of the path
Returns the path in the normal case
Returns the file part of path
Returns the directory part of path
commonprefix(list) Returns the longest common prefix of the paths in the list
gethome() Gets the home directory
getsize(path) Gets the size of the path file
Tests to see if path is absolute
Tests to see if path is a file
Tests to see if path is a dir
samepath(path1, path2) True if path1 and path2 represent the same file
join(list) Joins the path elements in the list
Returns (path, last_element)
Returns (drive, rest_of_path)
Returns (root, extension)

See the Python Library Reference for more information.

Appendix H: Regular expression control characters

The most useful Regular Expression special characters are:

Special Notation Comment(s)
Any character except those below Matches that character
. Matches any character
^ Matches the start of the string
$ Matches the end of the string
Matches longest 0 or 1 of the proceeding
Matches shortest 0 or 1 of the proceeding
Matches longest 1 or more of the proceeding
Matches shortest 1 or more of the proceeding
Matches longest 0 or more of the proceeding
Matches shortest 0 or more of the proceeding
Matches longest m to n of the proceeding
Matches shortest m to n of the proceeding
Defines the set of enclosed characters
Defines the set of non-enclosed characters
...|... Matches a choice (or)
Matches the sequence (or group) ...; groups are ordered from left to right with origin 1
Matches a sequence but does not define a group
Matches a sequence (or group) ... giving it a name
Matches the sequence defined with the name
Matches ... but does not consume the test
Matches not ... but does not consume the test
\c Special characters:
\c literal escapes: .?*+&^$|()[]
\c function escapes: see below

See the Python Library Reference for more information.

Function escapes:

Function Escapes Comment(s)
Matches at start of line
Matches at end of line
Matches not at beginning or end of a word
Matches at beginning or end of a word
Matches not any decimal digit (0..9)
Matches any decimal digit (0..9)
Matches not any white space
Matches any white space
Matches not any alpha-numeric characters
Matches any alpha-numeric characters
\# Matches group #

Several options exist to modify how regular expression are processed. Options are bit flags and may be combined by OR-ing (|) them together. Some of the more useful options are:

Option Comment(s)
-- or --
Match ignoring case
-- or --
Causes '^' and '$' to match internal line boundaries (vs. just the start and end of the string)
-- or --
Causes '.' to match a newline

Appendix I: Generated

The following is the code generated by jythonc compiler for the file of The factorial engine:

import org.python.core.*;

public class factor extends java.lang.Object {
     static String[] jpy$mainProperties =
         new String[] {"python.modules.builtin",

     static String[] jpy$proxyProperties =
         new String[] {"python.modules.builtin",

     static String[] jpy$packages = new String[] {};

     public static class _PyInner extends PyFunctionTable
            implements PyRunnable {
         private static PyObject i$0;
         private static PyObject i$1;
         private static PyObject s$2;
         private static PyObject l$3;
         private static PyObject i$4;
         private static PyObject s$5;
         private static PyObject s$6;
         private static PyObject s$7;
         private static PyObject s$8;
         private static PyObject s$9;
         private static PyObject i$10;
         private static PyObject i$11;
         private static PyObject s$12;
         private static PyFunctionTable funcTable;
         private static PyCode c$0___init__;
         private static PyCode c$1_addListener;
         private static PyCode c$2_addListeners;
         private static PyCode c$3_removeListener;
         private static PyCode c$4_removeListeners;
         private static PyCode c$5_fireListeners;
         private static PyCode c$6_cancel;
         private static PyCode c$7_calculate;
         private static PyCode c$8_Factorial;
         private static PyCode c$9_doFac;
         private static PyCode c$10_main;
         private static void initConstants() {
             i$0 = Py.newInteger(0);
             i$1 = Py.newInteger(1);
             s$2 = Py.newString("only positive integers supported: ");
             l$3 = Py.newLong("1");
             i$4 = Py.newInteger(100);
             s$5 = Py.newString("__main__");
             s$6 = Py.newString("running...");
             s$7 = Py.newString("For");
             s$8 = Py.newString("result =");
             s$9 = Py.newString("Exception -");
             i$10 = Py.newInteger(10);
             i$11 = Py.newInteger(1000);
             s$12 = Py.newString("C:\\Articles\\");
             funcTable = new _PyInner();
             c$0___init__ = Py.newCode(1, new String[] {"self"},
                                       "__init__", false, false,
                                       funcTable, 0,
                                       null, null, 0, 1);
             c$1_addListener = Py.newCode(2,
                                          new String[]
                                          {"self", "listener", "ll"},
                                          "addListener", false,
                                          false, funcTable, 1,
                                          null, null, 0, 1);
             c$2_addListeners = Py.newCode(2,
                                          new String[]
                                          {"self", "listeners", "l"},
                                          "addListeners", false,
                                          false, funcTable, 2,
                                          null, null, 0, 1);
             c$3_removeListener = Py.newCode(2,
                                          new String[]
                                          {"self", "listener", "ll"},
                                          "removeListener", false,
                                          false, funcTable, 3,
                                          null, null, 0, 1);
             c$4_removeListeners = Py.newCode(2,
                                          new String[]
                                          {"self", "listeners", "l"},
                                          "removeListeners", false,
                                          false, funcTable, 4,
                                           null, null, 0, 1);
             c$5_fireListeners = Py.newCode(2,
                                          new String[]
                                          {"self", "value", "func"},
                                          "fireListeners", false,
                                          false, funcTable, 5,
                                          null, null, 0, 1);
             c$6_cancel = Py.newCode(1,
                                          new String[]
                                          "cancel", false,
                                          false, funcTable, 6,
                                          null, null, 0, 1);
             c$7_calculate = Py.newCode(2,
                                          new String[]
                                          {"self", "value", "next",
                                           "x", "last", "result"},
                                          "calculate", false,
                                          false, funcTable, 7,
                                          null, null, 0, 1);
             c$8_Factorial = Py.newCode(0,
                                          new String[]
                                          "Factorial", false,
                                          false, funcTable, 8,
                                          null, null, 0, 0);
             c$9_doFac = Py.newCode(1,
                                          new String[]
                                          {"value", "e"},
                                          "doFac", false,
                                          false, funcTable, 9,
                                          null, null, 0, 1);
             c$10_main = Py.newCode(0,
                                          new String[] {},
                                          "main", false,
                                          false, funcTable, 10,
                                          null, null, 0, 0);

         public PyCode getMain() {
             if (c$10_main == null) _PyInner.initConstants();
             return c$10_main;

         public PyObject call_function(int index, PyFrame frame) {
             switch (index){
                 case 0:
                 return _PyInner.__init__$1(frame);
                 case 1:
                 return _PyInner.addListener$2(frame);
                 case 2:
                 return _PyInner.addListeners$3(frame);
                 case 3:
                 return _PyInner.removeListener$4(frame);
                 case 4:
                 return _PyInner.removeListeners$5(frame);
                 case 5:
                 return _PyInner.fireListeners$6(frame);
                 case 6:
                 return _PyInner.cancel$7(frame);
                 case 7:
                 return _PyInner.calculate$8(frame);
                 case 8:
                 return _PyInner.Factorial$9(frame);
                 case 9:
                 return _PyInner.doFac$10(frame);
                 case 10:
                 return _PyInner.main$11(frame);
                 return null;

         private static PyObject __init__$1(PyFrame frame) {
                            new PyList(new PyObject[] {}));
             frame.getlocal(0).__setattr__("_Factorial__cancelled", i$0);
             return Py.None;

         private static PyObject addListener$2(PyFrame frame) {
             if (frame.getlocal(1)._notin(
                    frame.getlocal(2) ).__nonzero__()) {
                 frame.getlocal(2).invoke("append", frame.getlocal(1));
             return Py.None;

         private static PyObject addListeners$3(PyFrame frame) {
             // Temporary Variables
             int t$0$int;
             PyObject t$0$PyObject, t$1$PyObject;

             // Code
             t$0$int = 0;
             t$1$PyObject = frame.getlocal(1);
             while ((t$0$PyObject =
                    t$1$PyObject.__finditem__(t$0$int++)) != null) {
                 frame.setlocal(2, t$0$PyObject);
             return Py.None;
                       private static PyObject removeListener$4(PyFrame frame) {
             frame.getlocal(2).invoke("remove", frame.getlocal(1));
             return Py.None;

         private static PyObject removeListeners$5(PyFrame frame) {
             // Temporary Variables
             int t$0$int;
             PyObject t$0$PyObject, t$1$PyObject;

             // Code
             t$0$int = 0;
             t$1$PyObject = frame.getlocal(1);
             while ((t$0$PyObject =
     t$1$PyObject.__finditem__(t$0$int++)) != null) {
                 frame.setlocal(2, t$0$PyObject);
             return Py.None;

         private static PyObject fireListeners$6(PyFrame frame) {
             // Temporary Variables
             int t$0$int;
             PyObject t$0$PyObject, t$1$PyObject;

             // Code
             t$0$int = 0;
             t$1$PyObject =
             while ((t$0$PyObject =
                     t$1$PyObject.__finditem__(t$0$int++)) != null) {
                 frame.setlocal(2, t$0$PyObject);
             return Py.None;

         private static PyObject cancel$7(PyFrame frame) {
             frame.getlocal(0).__setattr__("_Factorial__cancelled", i$1);
             return Py.None;

         private static PyObject calculate$8(PyFrame frame) {
             // Temporary Variables
             int t$0$int;
             PyObject t$0$PyObject, t$1$PyObject;

             // Code
             if (((t$0$PyObject = frame.getglobal("type").
                ? t$0$PyObject
                : frame.getlocal(1)._lt(i$0)).__nonzero__()) {
                 throw Py.makeException(
             frame.getlocal(0).__setattr__("_Factorial__cancelled", i$0);
             frame.setlocal(5, l$3);
             frame.getlocal(0).invoke("fireListeners", i$0);
             if (frame.getlocal(1)._le(i$1).__nonzero__()) {
                 frame.setlocal(5, l$3);
             else {
                 frame.setlocal(4, i$0);
                 t$0$int = 0;
                 t$1$PyObject = frame.getglobal("range").
                 while ((t$0$PyObject = t$1$PyObject.
                           __finditem__(t$0$int++)) != null) {
                     frame.setlocal(3, t$0$PyObject);
                     if (frame.getlocal(0).
                         __getattr__("_Factorial__cancelled").__nonzero__()) {
(frame.getlocal(2)._ne(frame.getlocal(4)).__nonzero__()) {
                         frame.setlocal(4, frame.getlocal(2));
             frame.getlocal(0).invoke("fireListeners", i$4);
             if (frame.getlocal(0).
                 __getattr__("_Factorial__cancelled").__nonzero__()) {
                 frame.setlocal(5, i$1.__neg__());
             return frame.getlocal(5);

         private static PyObject Factorial$9(PyFrame frame) {
                 new PyFunction(frame.f_globals,
                                new PyObject[] {}, c$0___init__));
                 new PyFunction(frame.f_globals,
                                new PyObject[] {}, c$1_addListener));
                 new PyFunction(frame.f_globals,
                                new PyObject[] {}, c$2_addListeners));
                 new PyFunction(frame.f_globals,
                                new PyObject[] {}, c$3_removeListener));
                 new PyFunction(frame.f_globals,
                                new PyObject[] {}, c$4_removeListeners));
                new PyFunction(frame.f_globals,
                               new PyObject[] {}, c$5_fireListeners));
                new PyFunction(frame.f_globals,
                               new PyObject[] {}, c$6_cancel));
                new PyFunction(frame.f_globals,
                               new PyObject[] {}, c$7_calculate));
             return frame.getf_locals();

         private static PyObject doFac$10(PyFrame frame) {
             // Temporary Variables
             PyException t$0$PyException;

             // Code
             try {
                    invoke("calculate", frame.getlocal(0)));
             catch (Throwable x$0) {
                 t$0$PyException = Py.setException(x$0, frame);
                 if (Py.matchException(t$0$PyException,
                                       frame.getglobal("ValueError"))) {
                     frame.setlocal(1, t$0$PyException.value);
                 else throw t$0$PyException;
             return Py.None;

         private static PyObject main$11(PyFrame frame) {
             frame.setglobal("__file__", s$12);

                           org.python.core.imp.importOne("sys", frame));
                           org.python.core.imp.importOne("types", frame));
                           org.python.core.imp.importOne("exceptions", frame));
                                        new PyObject[] {}, 
c$8_Factorial, null));
             if (frame.getname("__name__")._eq(s$5).__nonzero__()) {
                                new PyFunction(frame.f_globals,
                                               new PyObject[] {}, c$9_doFac));
             return Py.None;

     public static void moduleDictInit(PyObject dict) {
         dict.__setitem__("__name__", new PyString("factor"));
         Py.runCode(new _PyInner().getMain(), dict, dict);

     public static void main(String[] args) throws java.lang.Exception {
         String[] newargs = new String[args.length+1];
         newargs[0] = "factor";
         System.arraycopy(args, 0, newargs, 1, args.length);
         Py.runMain(factor._PyInner.class, newargs,
                    factor.jpy$mainProperties, null,
                    new String[] {"factor"});


Note: The above code has been reformatted for line length.

Appendix J: Formatting strings and values

Note that a simplified form of Appendix J originally appeared as multiple panels in Part 1 of this tutorial.

Jython strings support a special formating operation similar to C's printf, but using the modulo ("%") operator. The right-hand set of items is substituted into the left-hand string at the matching %x locations in the string. The set value is usually a single value, a tuple of values, or a dictionary of values.

The general format of the format specification is


Here's a guide to the format items:

  • key: Optional key to lookup in a supplied dictionary
  • flag: Optional flags (reasonable combinations supported)
    • #: Display any special format prefix (for example, "0" for octal, "0x" for hex)
    • +: Display a "+" on positive numbers
    • blank: Display a leading space on positive numbers
    • -: Left (vs. right) justify the value in the width
    • 0: Left pad with "0" (vs. spaces)
  • width: Minimum width of the field (will be longer for large values)
  • precision: Number of digits after any decimal point
  • x: Format code as described below

The format operator supports the following format characters:

Character(s) Result Format Comment(s)
%s, %r String %s does str(x), %r does repr(x)
%i, %d Integer Decimal Basically the same format
%o, %u, %x, %X Unsigned Value In octal, unsigned decimal, hexadecimal
%f, %F Floating Decimal Shows fraction after decimal point
%e, %E, %g, %G Exponential %g is %f unless the value is small; else %e
%c Character Must be a single character or integer
%% Character The % character

Note: more details on the structure and options of the format item can be found in the Python Library Reference (Related topics). Use of case in format characters (for example, X vs x causes the symbol to show in matching case.

For example

print "%s is %i %s %s than %s!" % ("John", 5, "years", "older", "Mark")

print "Name: %(last)s, %(first)s" % \
	{'first':"Barry", 'last':"Feigenbaum", 'age':18}


John is 5 years older than Mark!
Name: Feigenbaum, Barry

Appendix K: Built-in functions

Note that Appendix K appeared in Part 1 of this tutorial.

Jython provides very useful built-in functions that can be used without any imports. The most commonly used ones are summarized below:

Syntax Use/Comment(s) Example(s)
abs(x) Absolute value abs(-1) --> 1
apply(func, pargs {, kargs})
-- or --
func(*pargs {, **kargs})
Execute the function with the supplied positional arguments and optional keyword arguments apply(lambda x, y: x * y, (10, 20)) --> 200
callable(x) Tests to see if the object is callable (i.e, is a function, class or implements __call__) callable(MyClass) --> 1
chr(x) Converts the integer (0 - 65535) to a 1-character string chr(9) --> "\t"
cmp(x, y) Compares x to y: returns: negative if x < y; 0 if x == y; positive if x > y cmp("Hello", "Goodbye") --> > 0
coerce(x, y) Returns the tuple of x and y coerced to a common type coerce(-1, 10.2) --> (-1.0, 10.2)
compile(text, name, kind) Compile the text string from the source name. Kind is: "exec", "eval" or "single"
x = 2
c = compile("x * 2",
             "<string>", "eval")
eval(c) --> 4
complex(r, i) Create a complex number complex(1, 2) --> 1.0+2.0j
complex("1.0-0.1j") --> 1.0-0.1j
dir({namespace}) Returns a list of the keys in a namespace (local if omitted) dir() --> [n1, ..., nN]
vars({namespace}) Returns the namespace (local if omitted); do not change it vars() --> {n1:v1, ..., nN:vN}
divmod(x, y) Returns the tuple (x /y, x % y) divmod(100, 33) --> (3, 1)
eval(expr {, globals {, locals}}) Evaluate the expression in the supplied namespaces
myvalues = {'x':1, 'y':2}
eval("x + y", myvalues) --> 3
execfile(name {,globals {, locals}}) Read and execute the named file in the supplied namespaces execfile("")
filter(func, list) Creates a list of items for which func returns true filter(lambda x: x > 0, [-1, 0, 1, -5, 10]) --> [1, 10]
float(x) Converts x to a float float(10) --> 10.0
float("10.3") --> 10.3
getattr(object, name {, default}) Gets the value of the object's attribute; if not defined return default (or an exception if no default) getattr(myObj, "size", 0) --> 0
setattr(object, name, value) Creates/sets the value of the object's attribute setattr(myObj, "size", 10)
hasattr(object, name) Test to see if the object has an attribute hasattr(myObj, "size") --> 0
globals() Returns the current global namespace dictionary {n1:v1, ..., nN:vN}
locals() Returns the current local namespace dictionary {n1:v1, ..., nN:vN}
hash(object) Returns the object's hash value. Similar to java.lang.Object.hashCode() hash(x) --> 10030939
hex(x) Returns a hex string of x hex(-2) --> "FFFFFFFE"
id(object) Returns a unique stable integer id for the object id(myObj) --> 39839888
input(prompt) Prompts and evaluates the supplied input expression; equivalent to eval(raw_input(prompt)) input("Enter expression:")
with "1 + 2" --> 3
raw_input(prompt) Prompts for and inputs a string raw_input("Enter value:")
with "1 + 2" --> "1 + 2"
int(x{, radix}) Converts to an integer; radix: 0, 2..36; 0 implies guess int(10.2) --> 10
int("10") --> 10
int("1ff", 16) --> 511
isinstance(object, class) Tests to see if object is an instance of class or a subclass of class; class may be a tuple of classes to test multiple types isinstance(myObj, MyObject) --> 0
isinstance(x, (Class1, Class2)) --> 1
issubclass(xclass, clsss) Tests to see if xclass is a sub-(or same) class of class; class may be a tuple of classes to test multiple types issubclass(MyObject, (Class1, Class2)) --> 0
len(x) Returns the length (number of items) in the sequence or map len("Hello") --> 5
list(seq) Converts the sequence into a list list((1, 2, 3)) --> [1,2,3]
list("Hello") --> ['H','e','l','l','o']
tuple(seq) Converts the sequence into a tuple tuple((1, 2, 3)) --> (1,2,3) tuple("Hello")--> ('H','e','l','l','o')
long(x {, radix}) Converts to a long integer; radix: 0, 2..36; 0 implies guess long(10) --> 10L
long("10000000000") -->
map(func, list, ...) Creates a new list from the results of applying func to each element of each list map(lambda x,y: x+y, [1,2],[3,4]) --> [4,6]
map(None, [1,2],[3,4]) --> [[1,3],[2,4]]
max(x) Returns the maximum value max(1,2,3) --> 3
max([1,2,3]) --> 3
min(x) Returns the minimum value min(1,2,3) --> 1
min([1,2,3]) --> 1
oct(x) Converts to an octal string oct(10) --> "012
oct(-1) --> "037777777777"
open(name, mode {, bufsize}) Returns an open file. Mode is:(r|w|a){+}{b} open("useful.dat", "wb", 2048)
ord(x) Returns the integer value of the character ord('\t') --> 9
Computes x ** y
Computes x ** y % z
pow(2,3) --> 8
range({start,} stop {, inc})
xrange({start,} stop {, inc})
Returns a sequence ranging from start to stop in steps of inc; start defaults to 0; inc defaults to 1. Use xrange for large sequences (say more than 20 items) range(10) --> [0,1,2,3,4,5,6,7,8,9]
range(9,-1,-1) --> [9,8,7,6,5,4,3,2,1,0]
reduce(func, list {, init}) Applies func to each pair of items in turn accumulating a result reduce(lambda x,y:x+y, [1,2,3,4],5) --> 15
reduce(lambda x,y:x&y, [1,0,1]) --> 0
reduce(None, [], 1) --> 1
-- or --
Convert to a string from which it can be recreated, if possible repr(10 * 2) --> "'20'"
repr('xxx') --> "'xxx'"
x = 10; `x` --> "10'"
round(x {, digits}) Rounds the number round(10.009, 2) --> 10.01
round(1.5) --> 2
str(object) Converts to human-friendly string str(10 * 2) --> "20"
str('xxx') --> 'xxx'
type(object) Returns the type (not the same as class) of the object. To get the class use object.__class__. Module types has symbolic names for all Jython types x = "1"; type(x) is type('') --> 1
zip(seq, ...) Zips sequences together; results is only as long as the shortest input sequence zip([1,2,3],"abc") --> [(1,'a'),(2,'b'),(3,'c')]

See the Python Library Reference (Related topics) for more details.

Appendix L: Jython types summary

Note that Appendix L appeared in Part 1 of this tutorial.

Jython supports many object types. The module types defines symbols for these types. The function type gets the type of any object. The type value can be tested (see ). The table below summarizes the most often used types.

Type symbolJython runtime typeComment(s)
ArrayTypePyArrayAny array object
BuiltinFunctionTypePyReflectedFunctionAny built-in function object
BuiltinMethodTypePyMethodAny built-in method object
ClassTypePyClassAny Jython class object
ComplexTypePyComplexAny complex object
-- or --
PyDictionaryAny dictionary object
FileTypePyFileAny file object
FloatTypePyFloatAny float object
FunctionTypePyFunctionAny function object
InstanceTypePyInstanceAny class instance object
-- none --PyJavaInstanceAny Java class instance object
IntTypePyIntegerAny integer object
LambdaTypePyFunctionAny lambda function expression object
ListTypePyListAny list object
LongTypePyLongAny long object
MethodTypePyMethodAny non-built-in method object
ModuleTypePyModuleAny module object
NoneTypePyNoneAny None (only one) object
StringTypePyStringAny ASCII string object
TracebackTypePyTracebackAny exception traceback object
TupleTypePyTupleAny tuple object
TypeTypePyJavaClassAny type object
UnboundMethodTypePyMethodAny method (without a bound instancee) object
UnicodeTypePyStringAny Unicode string object
XRangeTypePyXRangeAny extended range object

Note: several types map to the same Java runtime type. For more information on types see the Python Library Reference (Related topics).

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Zone=Java development
ArticleTitle=Introduction to Jython, Part 2: Programming essentials