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Part 1 of this series looked at the basics of scripting
Kernel-based Virtual Machine (KVM) using
libvirt and Python. This installment uses the
concepts developed there to build several utility applications and add a
graphical user interface (GUI) into the mix. There are two primary options
for a GUI toolkit that has Python bindings and is cross-platform. The
first is Qt, which is now owned by Nokia; the second is wxPython. Both
have strong followings and many open source projects on their lists of
users.
For this article, I focus on wxPython more out of personal preference than
anything else. I start off with a short introduction to wxPython and the
basics proper setup. From there, I move on to a few short example
programs, and then to integrating with libvirt.
This approach should introduce enough wxPython basics for you to build a
simple program, and then expand on that program to add features.
Hopefully, you'll be able to take these concepts and build on them to meet
your specific needs.
A good place to start is with a few basic definitions. The wxPython library
is actually a wrapper on top of the C++-based
wxWidgets. In the context of creating a GUI, a widget is
essentially a building block. Five independent widgets reside at the
top-most level of the widget hierarchy:
wx.Frame,
wx.Dialog,
wx.PopupWindow,
wx.MDIParentFrame, and
wx.MDIChildFrame.
Most of the examples here are based on wx.Frame,
as it essentially implements a single modal window.
In wxPython, Frame is a class that you
instantiate as is or inherit from to add or enhance the functionality.
It's important to understand how widgets appear within a frame so you know
how to place them properly. Layout is determined either by absolute
positioning or by using sizers. A sizer is a handy tool that
resizes widgets when the user changes the size of the window by clicking
and dragging a side or corner.
The simplest form of a wxPython program must have a few lines of code to set things up. A typical main routine might look something like Listing 1.
Listing 1. Device XML definition
if __name__ == "__main__":
app = wx.App(False)
frame = MyFrame()
frame.Show()
app.MainLoop()
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Every wxPython app is an instance of wx.App()
and must instantiate it as shown in Listing 1. When you pass
False to wx.App, it
means "don't redirect stdout and stderr to a window." The next line
creates a frame by instantiating the MyFrame()
class. You then show the frame and pass control to
app.MainLoop(). The
MyFrame() class typically contains an
__init__ function to initialize the frame with
your widgets of choice. It is also where you would connect any widget
events to their appropriate handlers.
This is probably a good place to mention a handy debugging tool that comes with wxPython. It's called the widget inspection tool (see Figure 1) and only requires two lines of code to use. First, you have to import it with:
import wx.lib.inspection |
Then, to use it, you simply call the Show()
function:
wx.lib.inspectin.InspectionTool().Show() |
Clicking the Events icon on the menu toolbar dynamically shows you events as they fire. It's a really neat way to see events as they happen if you're not sure which events a particular widget supports. It also gives you a better appreciation of how much is going on behind the scenes when your application is running.
Figure 1. The wxPython widget inspection tool
Add a GUI to a command-line tool
Part 1 of this series presented a simple tool to display the
status of all running virtual machines (VMs). It's simple to change that
tool into a GUI tool with wxPython. The
wx.ListCtrl widget provides just the
functionality you need to present the information in tabular form. To use
a wx.ListCtrl widget, you must add it to your
frame with the following syntax:
self.list=wx.ListCtrl(frame,id,style=wx.LC_REPORT|wx.SUNKEN_BORDER) |
You can choose from several different styles, including the
wx.LC_REPORT and
wx.SUNKEN_BORDER options previously used. The
first option puts the wx.ListCtrl into Report
mode, which is one of four available modes. The others are Icon, Small
Icon, and List. To add styles like
wx.SUNKEN_BORDER, you simply use the pipe
character (|). Some styles are mutually
exclusive, such as the different border styles, so check the wxPython wiki
if you have any doubts (see Resources).
After instantiating the wx.ListCtrl widget, you
can start adding things to it, like column headers. The
InsertColumn method has two mandatory
parameters and two optional ones. First is the column index, which is
zero-based, followed by a string to set the heading. The third is for
formatting and should be something like
LIST_FORMAT_CENTER,
_LEFT, or _RIGHT.
Finally, you can set a fixed width by passing in an integer or have the
column automatically sized by using
wx.LIST_AUTOSIZE.
Now that you have the wx.ListCtrl widget
configured, you can use the InsertStringItem
and SetStringItem methods to populate it with
data. Each new row in the wx.ListCtrl widget
must be added using the InsertStringItem
method. The two mandatory parameters specify where to perform the insert,
with a value of 0 indicating at the top of the list and the string to
insert at that location. InsertStringItem
returns an integer indicating the row number of the inserted string. You
can make a call to GetItemCount() for the list
and use the return value for the index to append to the bottom, as Listing 2 shows.
Listing 2. GUI version of the command-line tool
import wx
import libvirt
conn=libvirt.open("qemu:///system")
class MyApp(wx.App):
def OnInit(self):
frame = wx.Frame(None, -1, "KVM Info")
id=wx.NewId()
self.list=wx.ListCtrl(frame,id,style=wx.LC_REPORT|wx.SUNKEN_BORDER)
self.list.Show(True)
self.list.InsertColumn(0,"ID")
self.list.InsertColumn(1,"Name")
self.list.InsertColumn(2,"State")
self.list.InsertColumn(3,"Max Mem")
self.list.InsertColumn(4,"# of vCPUs")
self.list.InsertColumn(5,"CPU Time (ns)")
for i,id in enumerate(conn.listDomainsID()):
dom = conn.lookupByID(id)
infos = dom.info()
pos = self.list.InsertStringItem(i,str(id))
self.list.SetStringItem(pos,1,dom.name())
self.list.SetStringItem(pos,2,str(infos[0]))
self.list.SetStringItem(pos,3,str(infos[1]))
self.list.SetStringItem(pos,4,str(infos[3]))
self.list.SetStringItem(pos,5,str(infos[2]))
frame.Show(True)
self.SetTopWindow(frame)
return True
app = MyApp(0)
app.MainLoop()
|
Figure 2 shows the results of these efforts.
Figure 2. The GUI KVM info tool
You can enhance the appearance of this table. A noticeable improvement
would be to resize the columns. You can do so by either adding the
width = parameter to the
InsertColumn call or use one line of code, like
this:
self.ListCtrl.SetColumnWidth(column,wx.LIST_AUTOSIZE) |
The other thing you could do is add a sizer so that the controls resize
with the parent window. You can do this with a
wxBoxSizer in a few lines of code. First, you
create the sizer, and then you add the widgets to it that you want to
resize along with the main window. Here's what that code might look like:
self.sizer = wx.BoxSizer(wx.VERTICAL) self.sizer.Add(self.list, proportion=1,flag=wx.EXPAND | wx.ALL, border=5) self.sizer.Add(self.button, flag=wx.EXPAND | wx.ALL, border=5) self.panel.SetSizerAndFit(self.sizer) |
The last call to self.panel.SetSizerAndFit()
instructs wxPython to set the initial size of the pane based on the
sizer's minimum size from the embedded widgets. This helps to give your
initial screen a reasonable size based on the content inside.
Control flow based on a user action
One of the nice things about the wx.ListCtrl
widget is that you can detect when a user clicks a specific part of the
widget and take some action based on that. This functionality allows you
to do things like sort a column alphabetically in forward or reverse order
based on the user clicking the column title. The technique to accomplish
this uses a callback mechanism. You must provide a function to handle each
action that you want to process by binding the widget and processing
method together. You do so with the Bind
method.
Every widget has some number of events associated with it. There are also
events associated with things like the mouse. Mouse events have names like
EVT_LEFT_DOWN,
EVT_LEFT_UP, and
EVT_LEFT_DCLICK, along with the same naming
convention for the other buttons. You could handle all mouse events by
attaching to the EVT_MOUSE_EVENTS type. The
trick is to catch the event in the context of the application or window
you're interested in.
When control passes to the event handler, it must perform the necessary steps to handle the action, and then return control to wherever it was prior to that. This is the event-drive programming model that every GUI must implement to handle user actions in a timely fashion. Many modern GUI applications implement multithreading to keep from giving the user the impression that the program isn't responding. I briefly touch on that later in this article.
Timers represent another type of event that a program must potentially deal with. For example, you might want to perform a periodic monitoring function at a user-defined interval. You would need to provide a screen on which the user could specify the interval, and then launch a timer that would in turn fire an event when it expires. The timer expiration fires an event that you can use to activate a section of code. You might need to set or restart the time, depending again on user preference. You could easily use this technique to develop a VM monitoring tool.
Listing 3 provides a simple demo app with a button and
static text lines. Using wx.StaticText is an
easy way to output a string to the window. The idea is to click the button
once to start a timer and record the start time. Clicking the button
records the start time and changes the label to Stop.
Clicking the button again fills in the stop time text box and changes the
button back to Start.
Listing 3. Simple app with a button and static text
import wx
from time import gmtime, strftime
class MyForm(wx.Frame):
def __init__(self):
wx.Frame.__init__(self, None, wx.ID_ANY, "Buttons")
self.panel = wx.Panel(self, wx.ID_ANY)
self.button = wx.Button(self.panel, id=wx.ID_ANY, label="Start")
self.button.Bind(wx.EVT_BUTTON, self.onButton)
def onButton(self, event):
if self.button.GetLabel() == "Start":
self.button.SetLabel("Stop")
strtime = strftime("%Y-%m-%d %H:%M:%S", gmtime())
wx.StaticText(self, -1, 'Start Time = ' + strtime, (25, 75))
else:
self.button.SetLabel("Start")
stptime = strftime("%Y-%m-%d %H:%M:%S", gmtime())
wx.StaticText(self, -1, 'Stop Time = ' + stptime, (25, 100))
if __name__ == "__main__":
app = wx.App(False)
frame = MyForm()
frame.Show()
app.MainLoop()
|
Now, you can add functionality to the simple monitoring GUI introduced
earlier. There is one more piece of wxPython you need to understand before
you have everything you need to create your app. Adding a check box to the
first column of a wx.ListCtrl widget would make
it possible to take action on multiple lines based on the status of the
check box. You can do this by using what wxPython calls mixins.
In essence, a mixin is a helper class that adds some type of
functionality to the parent widget. To add the check box mixin, simply use
the following code to instantiate it:
listmix.CheckListCtrlMixin.__init__(self) |
You can also take advantage of events to add the ability to select or clear all boxes by clicking the column title. Doing so makes it simple to do things like start or stop all VMs with just a few clicks. You need to write a few event handlers to respond to the appropriate events in the same way you changed the label on the button previously. Here's the line of code needed to set up a handler for the column click event:
self.Bind(wx.EVT_LIST_COL_CLICK, self.OnColClick, self.list) |
wx.EVT_LIST_COL_CLICK fires when any column
header is clicked. To determine which column was clicked, you can use the
event.GetColumn() method. Here's a simple
handler function for the OnColClick event:
def OnColClick(self, event):
print "column clicked %d\n" % event.GetColumn()
event.Skip()
|
The event.Skip() call is important if you need
to propagate the event to other handlers. Although this need might not be
apparent in this instance, it can be problematic when multiple handlers
need to process the same event. There's a good discussion of event
propagation on the wxPython wiki site, which has much more detail than I
have room for here.
Finally, add code to the two button handlers to start or stop all checked
VMs. It's possible to iterate over the lines in your
wx.ListCtrl and pull the VM ID out with just a
few lines of code, as Listing 4 shows.
Listing 4. Starting and stopping checked VMs
#!/usr/bin/env python
import wx
import wx.lib.mixins.listctrl as listmix
import libvirt
conn=libvirt.open("qemu:///system")
class CheckListCtrl(wx.ListCtrl, listmix.CheckListCtrlMixin,
listmix.ListCtrlAutoWidthMixin):
def __init__(self, *args, **kwargs):
wx.ListCtrl.__init__(self, *args, **kwargs)
listmix.CheckListCtrlMixin.__init__(self)
listmix.ListCtrlAutoWidthMixin.__init__(self)
self.setResizeColumn(2)
class MainWindow(wx.Frame):
def __init__(self, *args, **kwargs):
wx.Frame.__init__(self, *args, **kwargs)
self.panel = wx.Panel(self)
self.list = CheckListCtrl(self.panel, style=wx.LC_REPORT)
self.list.InsertColumn(0, "Check", width = 175)
self.Bind(wx.EVT_LIST_COL_CLICK, self.OnColClick, self.list)
self.list.InsertColumn(1,"Max Mem", width = 100)
self.list.InsertColumn(2,"# of vCPUs", width = 100)
for i,id in enumerate(conn.listDefinedDomains()):
dom = conn.lookupByName(id)
infos = dom.info()
pos = self.list.InsertStringItem(1,dom.name())
self.list.SetStringItem(pos,1,str(infos[1]))
self.list.SetStringItem(pos,2,str(infos[3]))
self.StrButton = wx.Button(self.panel, label="Start")
self.Bind(wx.EVT_BUTTON, self.onStrButton, self.StrButton)
self.sizer = wx.BoxSizer(wx.VERTICAL)
self.sizer.Add(self.list, proportion=1, flag=wx.EXPAND | wx.ALL, border=5)
self.sizer.Add(self.StrButton, flag=wx.EXPAND | wx.ALL, border=5)
self.panel.SetSizerAndFit(self.sizer)
self.Show()
def onStrButton(self, event):
if self.StrButton.GetLabel() == "Start":
num = self.list.GetItemCount()
for i in range(num):
if self.list.IsChecked(i):
dom = conn.lookupByName(self.list.GetItem(i, 0).Text)
dom.create()
print "%d started" % dom.ID()
def OnColClick(self, event):
item = self.list.GetColumn(0)
if item is not None:
if item.GetText() == "Check":
item.SetText("Uncheck")
self.list.SetColumn(0, item)
num = self.list.GetItemCount()
for i in range(num):
self.list.CheckItem(i,True)
else:
item.SetText("Check")
self.list.SetColumn(0, item)
num = self.list.GetItemCount()
for i in range(num):
self.list.CheckItem(i,False)
event.Skip()
app = wx.App(False)
win = MainWindow(None)
app.MainLoop()
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There are two things to point out here with respect to the state of VMs in
KVM: Running VMs show up when you use the
listDomainsID() method from
libvirt. To see non-running machines you must
use listDefinedDomains(). You just have to keep
those two separate so that you know which VMs you can start and which you
can stop.
This article focused mainly on the steps needed to build a GUI wrapper
using wxPython that in turn manages KVM with
libvirt. The wxPython library is extensive and
provides a wide range of widgets to enable you to build
professional-looking GUI-based applications. This article just scratched
the surface of its capabilities, but you'll hopefully be motivated to
investigate further. Be sure to check more Resources to help get your application running.
Learn
libvirtwebsite: Check out the entire site for more information.-
Reference Manual for
libvirt: Access the completelibvirtAPI reference manual. - Python.org: Find more of the Python resources you need.
-
wxPython.org: Get more about
wxPython.
- wxPython wiki: Expand your knowledge through the many tutorials
found here.
- developerWorks
Open source zone: Find extensive how-to information, tools, and
project updates to help you develop with open source technologies and use
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Paul Ferrill has been writing in the computer trade press for more than 20 years. He got his start writing networking reviews for PC Magazine on products like LANtastic and early versions of Novell Netware. Paul holds both BSEE and MSEE degrees and has written software for more computer platforms and architectures than he can remember.
