[ python-Bugs-834351 ] Mouse wheel crashes program

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Bugs item #834351, was opened at 2003-11-02 00:37
Message generated for change (Comment added) made by sdati
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Category: Tkinter
Group: Python 2.2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Gary Richardson (garyrxx)
Assigned to: Martin v. Löwis (loewis)
Summary: Mouse wheel crashes program

Initial Comment:
The following program (by Michael Peuser)  crashes as 
soon as the mouse wheel is moved. See my post to 
c.l.p. on Oct 29.

Gary Richardson

#-------------------------
from Tkinter import *
def _onMouseWheel(event):
    print event

root = Tk()
root.bind('<MouseWheel>',_onMouseWheel)
root.mainloop()


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Comment By: Steve Davis (sdati)
Date: 2005-02-08 02:40

Message:
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I have determined the root cause of this problem.  It actually does lie in 
Tk, but is only revealed by Python Tkinter because of the way Python 
deals with events.

>From Tk source -- tkWinX.c:

case WM_MOUSEWHEEL:
...
event.xany.send_event = -1
...

This line causes Tk to call TkpGetString() when ExpandPercents() is 
called with '%A' (which is done by Python for ALL events, including 
<MouseWheel>).  This, in turn, causes segmentation fault because 
event.xkey.trans_chars and event.xkey.nbytes are not initialized.

So, the workaround from the Python side is pretty ugly, but I have 
tested it and verified that it works.  There is probably a better way, but 
this was the obvious solution to me, since I have never needed to look at 
the Tkinter interface before:

In Tkinter.py, define a new _subst_format and _subst_format_str:

    _subst_format_workaround = ('%#', '%b', '%f', '%h', '%k',
             '%s', '%t', '%w', '%x', '%y',
             '%D', '%E', '%K', '%N', '%W', '%T', '%X', '%Y', '%D')
    _subst_format_str_workaround = " ".join(_subst_format_workaround)

Note that '%A' entry in _subst_format is replaced with '%D' entry in 
_subst_format_workaround.

Now, in Misc._bind(),

if sequence == "<MouseWheel>":
    cmd = ('%sif {"[%s %s]" == "break"} break\n'
           %
           (add and '+' or '',
            funcid, self._subst_format_str_workaround))
else:
    cmd = ('%sif {"[%s %s]" == "break"} break\n'
           %
           (add and '+' or '',
            funcid, self._subst_format_str))

I am submitting this bug to Tcl/Tk maintainers to request that they fix 
(Project: Tk Toolkit Request ID 1118340), but in the meantime (and for 
all older versions of Tk), it seems like a workaround in the Tkinter code 
might be appropriate.


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Comment By: John Speno (corvus)
Date: 2005-01-04 15:40

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I wasn't able to reproduce this in pure Tcl/Tk (under Wish). No surprise 
there. In wish, I used this:

toplevel .top
bind .top <MouseWheel> {puts 'foo'}

And mouse wheeling in the Toplevel widget made a bunch of 'foo's appear in 
the wish console without crashing.

HTH.


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Comment By: John Speno (corvus)
Date: 2005-01-04 15:02

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Me too. Python 2.3.4 with Tcl/Tk 8.4.9. Crash on Scroll Wheeling in WinXP 
SP 1. (users report it happening in SP 2 also).

I'll try to make this happen in pure Tcl/Tk rather than Tkinter too.

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Comment By: John Fouhy (johnfouhy)
Date: 2004-10-26 04:13

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I can reproduce this bug.  I am running Python 2.3.4 on
WinXP SP2.

tk84.dll is version 8.4.3; tcl84.dll is also 8.4.3.

The mousewheel works correctly with a Tkinter.Text object
with no additional bindings.


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Comment By: Jon Ashley (ash101)
Date: 2003-12-04 17:02

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Happens for me too, on Win2K with python 2.3.2. The fault is
in TCL84.DLL at an offset of 1002a58c, if that means
anything to anyone.

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Comment By: Gary Richardson (garyrxx)
Date: 2003-11-03 02:42

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I'm using Win98SE. 

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Comment By: Martin v. Löwis (loewis)
Date: 2003-11-02 18:37

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What operating system are you using?

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