Trying to unbind a specific event handler function from an event in Python Tkinter?

This is a very specific bug in Python Tkinter, where attempting to unbind a specific event handler function from a Tkinter widget’s event handler actually unbinds all the event handler functions.

It’s actually already documented, on the Old Python Bug Tracker #31485 / New Python Bug Tracker #75666 and on the Tkinter-Discuss Mailing List almost 10 years ago but it is still an issue in Python 3.11 and it took me a while to find so I’m documenting it here using the terms I first used to search for it. Amusingly, posting about bugs which took ages to find was one of the reasons I started this blog.

tkinter.Misc.unbind(self, sequence, funcid=None)

Anyway, the Python 3.11 code in /usr/lib/python3.11/tkinter/__init__.py reads:

    def unbind(self, sequence, funcid=None):
        """Unbind for this widget for event SEQUENCE  the
        function identified with FUNCID."""
        self.tk.call('bind', self._w, sequence, '')
        if funcid:
            self.deletecommand(funcid)

Which, well, it definitely does unbind the function identified by funcid but it also unbinds all the other functions too. Which is pretty confusing if you had multiple handlers listening for the same event.

The underlying implementation (in _bind) is pretty confusing, but suffice to say your call to bind is somehow writing a little bit of Tcl/Tk code and attaching that to the widget, and your call to unbind is clearing it out again.

A fixed unbind

To delete a specific handler you have to work out where it is in the Tcl/Tk code, and there’s a few implementations in the issues linked above but here’s mine which uses a regular “now you’ve got two problems” expression to fix the bound code:

def unbind(widget, seq, funcid):
    # widget.unbind(seq, funcid) doesn't actually work as documented. This is
    # my own take on the horrible hacks found at https://bugs.python.org/issue31485
    # and https://github.com/python/cpython/issues/75666 and
    # https://mail.python.org/pipermail/tkinter-discuss/2012-May/003152.html
    # Also quite horrible.  "I'm not proud, but I'm not tired either"

    widget.bind(seq, re.sub(
        r'^if {"\[' + funcid + '.*$', '', widget.bind(seq), flags=re.M
    ))
    widget.deletecommand(funcid)

… you could make this into a FixedUnbindMixin class I suppose, or make like a Ruby programmer and monkey patch it until such time as a fix is merged into tkinter.

Also note that if you call unbind without a funcid (or call bind without add=True and that happens to write over an existing handler) that previously registered command is never collected, which is maybe not great if you do it a lot.

Perhaps the reason this bug has lain dormant for so long is that it’d be better to just bind events on a parent object instead?