Jython exceptions

About exceptions

Regardless of how much care a programmer takes in designing and testing his or her code, unexpected errors, or exceptions, can occur. Jython provides excellent support for recovering from these errors,

Exceptions are generally subclasses of the Jython type exceptions.Exception or the Java class java.lang.Exception. Most Jython exception names end in "Error" (such as IOError or IndexError) or "Warning." Java exceptions end in either "Error" (for critical exceptions) or "Exception" (for generally recoverable exceptions). For more information see The Jython exception hierarchy or the Python Library Reference (see Resources for a link).

The Jython exception hierarchy

Here is Jython's principle exception hierarchy subset.

• 1 Exception
  • 1.1 SystemExit
  • 1.2 StopIteration
  • 1.3 StandardError
    • 1.3.1 KeyboardInterrupt
    • 1.3.2 ImportError
    • 1.3.3 EnvironmentError
      • 1.3.3.1 IOError
      • 1.3.3.2 OSError
    • 1.3.4 EOFError
    • 1.3.5 RuntimeError
      • 1.3.5.1 NotImplementedError
    • 1.3.6 NameError
      • 1.3.6.1 UnboundLocalError
    • 1.3.7 AttributeError
    • 1.3.8 SyntaxError
      • 1.3.8.1 IndentationError
      • 1.3.8.2 TabError
    • 1.3.9 TypeError
    • 1.3.10 AssertionError
    • 1.3.11 LookupError
      • 1.3.11.1 IndexError
      • 1.3.11.2 KeyError
    • 1.3.12 ArithmeticError
      • 1.3.12.1 OverflowError
      • 1.3.12.2 ZeroDivisionError
      • 1.3.12.3 FloatingPointError
    • 1.3.13 ValueError
    • 1.3.14 ReferenceError
    • 1.3.15 SystemError
    • 1.3.16 MemoryError
• 2 Warning
  • 2.1 UserWarning
  • 2.2 DeprecationWarning
  • 2.3 PendingDeprecationWarning
  • 2.4 SyntaxWarning
  • 2.5 OverflowWarning
  • 2.6 RuntimeWarning
  • 2.7 FutureWarning

This hierarchy is a subset of the Python Library Reference (see Resources). These exceptions may be subclassed.

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The try-except-else statement

Like C++ and the Java language, Jython supports exception handlers. These handlers are defined by the try-except-else statement, which has the following form:

try: statement
except type, var: statement
  :
else: statement

-- or --

try:
    block
except type, var:
    block
  :
else:
    block
                

The except clause may be repeated with different type values. If so, the exceptions either must not overlap hierarchically (that is, be siblings) or they must be ordered from child to root exceptions. The optional type value is an exception type (either a subclass of exceptions.Exception or java.lang.Throwable). If type is missing, then the except clause catches all Jython and Java exceptions. The optional var value receives the actual exception object. If var is missing, then the exception object is not directly accessible. The else clause is optional. It is executed only if no exception occurs.

If an exception occurs in the try clause, the clause is exited and the first matching except clause (if any) is entered. If no exception matches, the block containing the try-except-else is exited and the exception is re-raised.

If an exception is raised in the except or else clause, the clause will exit and the new exception will be processed in the containing block.

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Accessing exception information

To access information about an exception, you may use the value provided in the except clause as described previously or the sys.exc_info function. For example, you can use the following function, in which type is the class of the exception, value is the exception object (use str(value) to get the message), and traceback is the execution trace back, which is a linked list of execution stack frames.

import sys
  :
try:
     :
except:
     type, value, traceback = sys.exc_info()

More details on the exceptions and trace backs is available in the Python Reference Manual (see Resources).

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The try-finally statement

Like C++ and the Java language, Jython supports an additional construct, try-finally, which makes it easy to do required cleanup activities such as closing open files, releasing resources, etc. Any code in the finally clause is guaranteed to be executed once the try clause is entered, even if it is exited via a return statement (see The return statement) or an exception. The try-finally statement has the following forms:

try: statement
finally: statement

  -- or --

try:
    block
finally:
    block
                

Note that try-except-else statements may nest in try-finally statements and vice versa.

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A try statement example

Here is an example of using both try-except and try-finally statements together. We'll talk more about Jython file I/O in Part 2 of this tutorial.

def readfile (name):
     "return the lines in a file or None if the file cannot be read"
     try:
         file = open(name, 'r')  # access the file
         try:
             return file.readlines()
         finally:
             file.close()    # ensure file is closed
     except IOError, ioe:    # report the error
         print "Exception -", ioe

:

# prints Exception - File not found - nofile (...)
# then None
print readfile("nofile")      

# prints a list of the lines in the file
print readfile("realfile")    

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The raise statement

Exceptions are generated by called functions or built-in services. You can also generate one by using the raise statement. The raise statement has the following forms:

raise exception

  -- or --

raise exception_class {, message}

  -- or --

raise

Below are some example raise statements.

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Table of contents

• About this tutorial
• Getting started
• Basic concepts and advantages of Jython
• Working with Jython
• Jython basic data types
• Jython collections
• Jython program structure
• Modules and importing
• Jython exceptions
• Jython procedural statements
• Jython functions
• Wrap-up
• Appendices
• Download
• Resources
• About the author

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