Found when trying to answer this SO question: https://stackoverflow.com/questions/53171384/tkinter-function-repeats-itself-twice-when-ttk-widgets-are-engaged

The ttk.OptionMenu uses radiobuttons for the dropdown menu, whereas the tkinter.OptionMenu uses commands. Both of them use `_setit` as command, presumably first used for tkinter and then copied to the ttk implementation. The `_setit` does two things: changing the associated `variable` and calling the associated callback function. This is needed for the tkinter.OptionMenu since commands don't support changing a `variable`.

However, for the ttk.OptionMenu an additional reference to the variable was added to the radiobutton following this issue: https://bugs.python.org/issue25684. This was needed to group radiobuttons, but now leads to the variable being changed twice: once by the radiobutton and once through `_setit`. When tracing the variable this leads to the tracing callback being called twice on only one change of the OptionMenu.

The solution is to not use `_setit` for the radiobutton command but instead just use `command=self._callback`

Thank you for reporting this issue (and doing your research!). I don't think we could use `command=self._callback` since this wouldn't pass the new value, but something like `command=lambda val=val: self._callback(val)` would work perfectly. Would you like to open a pull request for this (with accompanying news entry and test), or would you rather I did it?

Ah yes, you're right that `command=self._callback` doesn't pass the new value like it should.
Your suggestion does, but throws a "TypeError: 'NoneType' object is not callable" when no command is given when creating the OptionMenu. The current `_setit` implementation uses a `if self.__callback:` check for that.

Should we do something like this?

for val in values:
    menu.add_radiobutton(label=val,
        variable=self._variable)
    if self._callback:
        menu.entryconfigure('last', command=lambda val=val: self._callback(val))


I don't have any experience making pull requests. I might look into it at some point but for now it would be great if you could make it.

New changeset add46f84769a7e6fafa50954f79b7c248231fa4e by E-Paine in branch 'main':
bpo-45160: Ttk optionmenu only set variable once (GH-28291)
https://github.com/python/cpython/commit/add46f84769a7e6fafa50954f79b7c248231fa4e

New changeset 04485ac9886cd807aae854fb905a88c791cfdde6 by Miss Islington (bot) in branch '3.9':
bpo-45160: Ttk optionmenu only set variable once (GH-28291) (GH-29132)
https://github.com/python/cpython/commit/04485ac9886cd807aae854fb905a88c791cfdde6

New changeset 98f157de1260801fd26c72eb6bfae0d9295e5ab3 by Miss Islington (bot) in branch '3.10':
bpo-45160: Ttk optionmenu only set variable once (GH-28291) (GH-29131)
https://github.com/python/cpython/commit/98f157de1260801fd26c72eb6bfae0d9295e5ab3

Thanks for the fix, E. Paine, and fhdrsdg for the report! ✨ 🍰 ✨

New changeset 54d1e3f72ed1ad8e860888c30ee7a285b931c0d1 by Łukasz Langa in branch 'main':
bpo-45160: Fix refleak in test_ttk_guionly introduced in GH-28291 (GH-29416)
https://github.com/python/cpython/commit/54d1e3f72ed1ad8e860888c30ee7a285b931c0d1

New changeset 276a3a6a16ebf489607db31ad86c58dd89567306 by Miss Islington (bot) in branch '3.9':
bpo-45160: Fix refleak in test_ttk_guionly introduced in GH-28291 (GH-29416) (GH-29420)
https://github.com/python/cpython/commit/276a3a6a16ebf489607db31ad86c58dd89567306

New changeset 099a94fba3f9437e29d16ed54215ec8cf65de868 by Miss Islington (bot) in branch '3.10':
bpo-45160: Fix refleak in test_ttk_guionly introduced in GH-28291 (GH-29416) (GH-29421)
https://github.com/python/cpython/commit/099a94fba3f9437e29d16ed54215ec8cf65de868