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Name: [002] Ian Ozsvald
Member: 104 months
Authored: 181 videos
Description: I am the co-founder of ShowMeDo (see http://showmedo.com/about), author of `The Screencasting Handbook <http://thescreencastinghandbook.com>`_ and the founder of the professional screencast production company `ProCasts <http://procasts.co.uk>`_: .. image:: http://procasts.co.uk/media/procasts_sma ...

Avoiding Memory Leaks with .Destroy() [ID:541] (8/14)

in series: Build a wxPython Image Viewer

(Showmedo is undergoing major changes. To report any problems viewing the videos please email us and include browser and OS specifics. Cheers - Kyran.)

Memory leaks lead to insidious bugs - here we look at a memory leak which exists in the code, inspect the problem and then remove the leak.

The memory leak exists because we add a new StaticBitmap to a Panel and we don't remove the prior StaticBitmap from the previous call. The Panel keeps track of a growing list of StaticBitmaps which we don't need - these constitute a 'leak' (though they don't actually cause us any problems). It is always good programming practice to avoid memory leaks.

We ask the Panel to GetChildren (inherited from Window.GetChildren) and show how many children are being tracked. We then use .Destroy() (inherited from Window.Destory()) to safely clean-up the previous StaticBitmap.

import os
import wx

MAIN_WINDOW_DEFAULT_SIZE = (300,200)

class Frame(wx.Frame):
    
    def __init__(self, parent, id, title):
        style=wx.DEFAULT_FRAME_STYLE ^ (wx.RESIZE_BORDER) # XOR to remove the resizeable border        
        wx.Frame.__init__(self, parent, id, title=title, size=MAIN_WINDOW_DEFAULT_SIZE, style=style)
        self.Center() # open in the centre of the screen
        self.panel = wx.Panel(self)
        self.panel.SetBackgroundColour('White') # make the background of the window white

        self.CreateMenuBar()
        
        # create a StatusBar and give it 2 columns
        self.statusBar = self.CreateStatusBar()
        self.statusBar.SetFieldsCount(2)
        self.statusBar.SetStatusText('No image specified', 1)
        
        self.bitmap = None # set to None as we refer to it in ShowBitmap before we instantiate it
        

    def CreateMenuBar(self):
        "Create a menu bar with Open, Exit items"
        menuBar = wx.MenuBar()
        # Tell our Frame about this MenuBar
        self.SetMenuBar(menuBar)
        menuFile = wx.Menu()
        menuBar.Append(menuFile, '&File')
        # NOTE on wx ids - they're used everywhere, we don't care about them
        # Used to handle events and other things
        # An id can be -1 or wx.ID_ANY, wx.NewId(), your own id
        # Get the id using object.GetId()
        fileOpenMenuItem = menuFile.Append(-1, '&Open Image', 'Open a picture')
        #print "fileOpenMenuItem.GetId()", fileOpenMenuItem.GetId()
        self.Bind(wx.EVT_MENU, self.OnOpen, fileOpenMenuItem)

        # add a 'mirror' option, disable it for now
        # we add mirrorMenuItem to self so that we can reference it later
        #self.mirrorMenuItem = menuFile.Append(-1, '&Mirror Image', 'Mirror the image horizontally')
        #self.mirrorMenuItem.Enable(False) # we can't mirror an image until we've loaded one in, so start with 'mirror' disabled
        #self.Bind(wx.EVT_MENU, self.OnMirrorImage, self.mirrorMenuItem)
        
        # create a menu item for Exit and bind it to the OnExit function       
        exitMenuItem = menuFile.Append(-1, 'E&xit', 'Exit the viewer')        
        self.Bind(wx.EVT_MENU, self.OnExit, exitMenuItem)
        
        # add a Help menu with an About item
        #menuHelp = wx.Menu()
        #menuBar.Append(menuHelp, '&Help')
        #helpMenuItem = menuHelp.Append(-1, '&About', 'About screen')
        #self.Bind(wx.EVT_MENU, self.OnAbout, helpMenuItem)

    def OnOpen(self, event):
        "Open an image file, set title if successful"
        # Create a file-open dialog in the current directory
        
        filters = 'Image files (*.gif;*.png;*.jpg)|*.gif;*.png;*.jpg'
        dlg = wx.FileDialog(self, message="Open an Image...", defaultDir=os.getcwd(), 
                            defaultFile="", wildcard=filters, style=wx.OPEN)
        
        # Call the dialog as a model-dialog so we're required to choose Ok or Cancel
        if dlg.ShowModal() == wx.ID_OK:
            # User has selected something, get the path, set the window's title to the path
            filename = dlg.GetPath()
            self.SetTitle(filename)
            wx.BeginBusyCursor()            
            # load the image from the filename
            self.image = wx.Image(filename, wx.BITMAP_TYPE_ANY, -1) # auto-detect file type        
            # set the StatusBar to show the image's size
            self.statusBar.SetStatusText('Size = %s' % (str(self.image.GetSize())) , 1)
            # display the image inside the panel
            self.ShowBitmap()
            wx.EndBusyCursor()
                        
        dlg.Destroy() # we don't need the dialog any more so we ask it to clean-up

    def ShowBitmap(self):
        # NOTE doesn't delete old bitmap which can cause a memory leak!
        window_list = self.panel.GetChildren()
        print "window_list", len(window_list) # increases with each new item
        ### window_list[n].Destroy() # could delete items by iterating over the list
        #if self.bitmap is not None:
        #    self.bitmap.Destroy()
        #window_list = self.panel.GetChildren()
        #print "window_list after Destroy", len(window_list) 
        
        # Convert to Bitmap for wxPython to draw it to screen
        self.bitmap = wx.StaticBitmap(self.panel, -1, wx.BitmapFromImage(self.image))       
        # Make the application's window as large as the image
        self.SetClientSize(self.bitmap.GetSize())
        self.Center() # open in the centre of the screen
        
    def OnExit(self, event):
        "Close the application by Destroying the object"
        self.Destroy() 
        
    
class App(wx.App):
    
    def OnInit(self):
        self.frame = Frame(parent=None, id=-1, title='Image Viewer')
        self.frame.Show()
        self.SetTopWindow(self.frame)
        return True
    
if __name__ == "__main__":       
    # make an App object, set stdout to the console so we can see errors
    app = App(redirect=False)
        
    app.MainLoop()

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All comments excluding tick-boxed quick-comments

Thank you, I had a problem with deleting the old bitmaps stored by my program. As its intention is to be open while the user opens and closes several images.


Great videos. thanks a lot.

in the fuction ShowBitmap we use the if statement "if self.bitmap is not None", Why would we use "is not" over "!="


Definitely learned something new with this video; it took watching a couples times and playing around with, but I got the just of it :-)

Commenting / Uncommenting lines out is a great way to see just what each line does and why it's there; I really have fun figuring out ways to sabotage my program :-)

You're right doing is the best way to learn!


Of course you could have used a try/except block :-) It would have meant me introducing 2 concepts here and I try to do just 1 at a time.

Ian.


Gr8. I did

if self.bitmap is not None:

self.bitmap.Destroy()

window_list = self.panel.GetChildren()

print "window_list after Destroy", len(window_list)

so it is only executed when needed, could it have been a try and no self.bitmap = None

or is that 2 advanced for this part?


Hey Ian.

Yet another very nice & useful episode.

The explanation of destroying objects and why you are supposed to destroy them was very clear.

It's nice that you also show these kind of things, and point it out clearly, even if the application is small.

Thanks


Hi Marlow. Thanks for the feedback and yes, I hope the feeling of 'overcomplicatedness' lessens too :-)

Cheers,

Ian.


Nice episode. I enjoyed learning a bit about wxpython in this series so far--after the initial exposure to your python on xp. Keep the wxpython episodes coming, but for now I'd best go back to python proper & learn more there.

Everything seems both familiar (e.g. yeah, we do that in xy language) and alien (terminology, naming conventions, etc.). It also seems overcomplicated simply because it's still so new to me. These episodes do a nice job of leading the newbie along & developing some confidence. Guess I'll have more confidence after doing my own first little app. I do hope the overcomplicated feeling lessens then!

Thanks Ian!


Video published, thanks for contributing to ShowMeDo


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