UnicodeDecodeError in OS X tkinter when binding to <MouseWheel>

[culler]

BPO	10731
Nosy	@ronaldoussoren, @ned-deily

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ned-deily'
closed_at = <Date 2012-08-02.07:58:12.508>
created_at = <Date 2010-12-18.15:22:39.788>
labels = ['OS-mac', 'type-bug', 'expert-tkinter']
title = 'UnicodeDecodeError in OS X tkinter when binding to <MouseWheel>'
updated_at = <Date 2014-05-23.03:55:54.555>
user = 'https://bugs.python.org/culler'

bugs.python.org fields:

activity = <Date 2014-05-23.03:55:54.555>
actor = 'Dougal.Graham'
assignee = 'ned.deily'
closed = True
closed_date = <Date 2012-08-02.07:58:12.508>
closer = 'ned.deily'
components = ['macOS', 'Tkinter']
creation = <Date 2010-12-18.15:22:39.788>
creator = 'culler'
dependencies = []
files = []
hgrepos = []
issue_num = 10731
keywords = []
message_count = 12.0
messages = ['124287', '140666', '141052', '158936', '158946', '158947', '158966', '159050', '167198', '167242', '218941', '218943']
nosy_count = 6.0
nosy_names = ['ronaldoussoren', 'wordtech', 'ned.deily', 'culler', 'Marc.Culler', 'Dougal.Graham']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue10731'
versions = ['Python 3.2', 'Python 3.3']

[culler]

In OS X tkinter the "two-finger scroll" generates <MouseWheel> events.

The following code:

#### bug.py
import tkinter

def mouse_wheel(event):
    print('Mouse wheel event')

tk = tkinter.Tk()
list = tkinter.Listbox(tk)
list.bind('<MouseWheel>', mouse_wheel)
for n in range(20):
    list.insert(tkinter.END, str(n**n))
list.pack(fill=tkinter.BOTH, expand=1)
tk.mainloop()
####

often throws a UnicodeDecodeError exception with a traceback like the one shown below when you do a "two-finger scroll" in the window.

Traceback (most recent call last):
  File "bug.py", line 12, in <module>
    tk.mainloop()
  File "/Library/Frameworks/Python.framework/Versions/3.1/lib/python3.1/tkinter/__init__.py", line 1009, in mainloop
    self.tk.mainloop(n)
UnicodeDecodeError: 'utf8' codec can't decode byte 0x84 in position 1: unexpected code byte

The value of the byte changes from time to time, and sometimes it may be possible to scroll a few times without producing the error.

This problem did not occur on my Mandriva 2010.2 system. I think it is specific to the Macintosh. It might possibly be related to:
http://bugs.python.org/issue834351

[ronaldoussoren]

This is almost certainly a bug in Tk. What OSX version are you using, and which python installer did you use?

One thing you could try is installing a matching copy of ActiveState's Tk (8.4 for the 32-bit build, 8.5 for intel-only builds of python 2.7 and 3.2)

[culler]

I am running OSX 10.5.8 on this macbook. The Tcl/Tk package on the system is ActiveState Tcl/Tk 8.4.19.

I just installed Python 3.2 (r32:88452, Feb 20 2011, 10:19:59) from http://www.python.org/getit/releases/3.2/ and I am still seeing this bug. It does not occur with the Python 2.7 Tkinter package, which uses the same Tcl/Tk framework. So if it is a bug in Tk, it is harmless when used with Python 2.7. But, after all, it is not so surprising to see new UnicodeDecodeErrors when porting from Python 2 to Python 3.

[ned-deily]

The exception also occurs with Python 3.3 linked with Cocoa Tk 8.5. However, the it does not appear when Python 3.x is linked with Carbon Tk 8.4.

[ned-deily]

It looks like the problem is that the current Cocoa Tcl/Tk 8.5.x returns an incorrect MouseWheel event. Using the supplied test program and breakpointing in PythonCmd (around Modules/_tkinter.c:2027 in default), I found that it is being called from Tcl for MouseWheel events with an "argc" of 20, which looks suspiciously like the length of argv[1], "4302153816mouse_wheel", rather than the number of arguments which should be more like 3. It may also be an issue that affects Python because _tkinter still uses the older Tcl_CreateCommand interface rather than the newer Tcl_CreateObjCommand. The same Tcl behavior is observed with Python 2.7 _tkinter.c but there the bogus arguments are translated using PyString_FromString which is unaffected by the garbage arguments. It might be possible to workaround this problem in _tkinter but the next step is to open a Tcl/Tk issue against the Cocoa implementation and push for a proper fix there.

http://www.tcl.tk/man/tcl8.5/TclLib/CrtCommand.htm