Code without effect (wx demo TreeCtrl.py ImageList)

[Martin Drautzburg]

In the wx demoy TreeCtrl.py I find the following code, that should
have no effect but seems to be needed nevertheless.

class TestTreeCtrlPanel(wx.Panel):
def __init__(self, parent, log):
[...}
self.tree = MyTreeCtrl(self, tID, wx.DefaultPosition, ...
isz = (16,16)
il = wx.ImageList(isz[0], isz[1])
fldridx = il.Add(wx.ArtProvider_GetBitmap(wx.ART_FOLDER, wx.ART...
[...]

self.tree.SetImageList(il)
--> self.il = il

What is the effect of the last statement? self.il is not used
anywhere. I used similar code in my application and it crashes unless
I assign the image list to the parent panel. The name of the attribute
does not seem to matter. I can write self.foo=il just as well, but
without it it crashes.

 

[Steve Holden]

In the wx demoy TreeCtrl.py I find the following code, that should
have no effect but seems to be needed nevertheless.

class TestTreeCtrlPanel(wx.Panel):
def __init__(self, parent, log):
[...}
self.tree = MyTreeCtrl(self, tID, wx.DefaultPosition, ...
isz = (16,16)
il = wx.ImageList(isz[0], isz[1])
fldridx = il.Add(wx.ArtProvider_GetBitmap(wx.ART_FOLDER, wx.ART...
[...]

self.tree.SetImageList(il)
--> self.il = il

What is the effect of the last statement? self.il is not used
anywhere. I used similar code in my application and it crashes unless
I assign the image list to the parent panel. The name of the attribute
does not seem to matter. I can write self.foo=il just as well, but
without it it crashes.

You will probably find that without that reference to the image list it
will be garbage-collected while the images are still beign displayed,
leading to results that are at best unpredictable and at worst fatal to
your program.

regards
Steve

 

[Paul McGuire]

In the wx demoy TreeCtrl.py I find the following code, that should
have no effect but seems to be needed nevertheless.

class TestTreeCtrlPanel(wx.Panel):
def __init__(self, parent, log):
[...}
self.tree = MyTreeCtrl(self, tID, wx.DefaultPosition, ...
isz = (16,16)
il = wx.ImageList(isz[0], isz[1])
fldridx = il.Add(wx.ArtProvider_GetBitmap(wx.ART_FOLDER, wx.ART...
[...]

self.tree.SetImageList(il)
--> self.il = il

What is the effect of the last statement? self.il is not used
anywhere. I used similar code in my application and it crashes unless
I assign the image list to the parent panel. The name of the attribute
does not seem to matter. I can write self.foo=il just as well, but
without it it crashes.

Two possibilities come to mind:
- used by a base class wx.Panel for which you do not have the source
(although if this were the case, I would think the attribute name would be
significant)
- needed to retain a reference handle to the image list, to prevent it from
being garbage collected (kind of far-fetched, but consistent with the "name
it anything you want" behavior)

-- Paul