tkinter focus_get() with non-tkinter Tk widget

[Akuli]

BPO	44592
Nosy	@terryjreedy, @serhiy-storchaka, @Akuli, @E-Paine
Superseder	bpo-734176: Make Tkinter.py's nametowidget work with cloned menu widgets

^{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 = None
closed_at = None
created_at = <Date 2021-07-09.15:14:18.546>
labels = ['type-bug', 'expert-tkinter', '3.9', '3.10', '3.11']
title = 'tkinter focus_get() with non-tkinter Tk widget'
updated_at = <Date 2021-07-17.18:19:16.891>
user = 'https://github.com/Akuli'

bugs.python.org fields:

activity = <Date 2021-07-17.18:19:16.891>
actor = 'Akuli'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tkinter']
creation = <Date 2021-07-09.15:14:18.546>
creator = 'Akuli'
dependencies = []
files = []
hgrepos = []
issue_num = 44592
keywords = []
message_count = 12.0
messages = ['397199', '397200', '397211', '397213', '397216', '397217', '397220', '397222', '397224', '397681', '397726', '397727']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'serhiy.storchaka', 'Akuli', 'epaine']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'open'
superseder = '734176'
type = 'behavior'
url = 'https://bugs.python.org/issue44592'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Linked PRs

• gh-88758: Handle non-tkinter widgets in tkinter focus methods #152337
• [3.15] gh-88758: Handle non-tkinter widgets in tkinter focus methods (GH-152337) #152346
• [3.14] gh-88758: Handle non-tkinter widgets in tkinter focus methods (GH-152337) #152347
• [3.13] gh-88758: Handle non-tkinter widgets in tkinter focus methods (GH-152337) #152348

[Akuli]

The purpose of focus_get() is to return the widget that currently has the focus. It tries to convert its result to a tkinter widget, which can fail, because not all Tk widgets are known to tkinter. Consider this, for example:

    import tkinter

    def print_focused_widget():
        print(repr(root.focus_get()))
        root.after(1000, print_focused_widget)

    root = tkinter.Tk()
    menu = root["menu"] = tkinter.Menu()
    menu.add_cascade(label="Click here", menu=tkinter.Menu())
    print_focused_widget()
    tkinter.mainloop()

Output, with menu clicked after a couple seconds (on Linux):

    None
    <tkinter.Tk object .>
    <tkinter.Tk object .>
    Exception in Tkinter callback
    Traceback (most recent call last):
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 1916, in __call__
        return self.func(*args)
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 838, in callit
        func(*args)
      File "/home/akuli/porcu/foo.py", line 4, in print_focused_widget
        print(repr(root.focus_get()))
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 782, in focus_get
        return self._nametowidget(name)
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 1531, in nametowidget
        w = w.children[n]
    KeyError: '#!menu'

Some nametowidget() calls in tkinter/init.py already handle this correctly. Consider winfo_children(), for example:

        try:
            # Tcl sometimes returns extra windows, e.g. for
            # menus; those need to be skipped
            result.append(self._nametowidget(child))
        except KeyError:
            pass

[Akuli]

Forgot to mention: The correct fix IMO would be to return None when a KeyError occurs. This way code like focus_get() == some_tkinter_widget would always do the right thing, for example.

[E-Paine]

I agree with Akuli that raising a KeyError is not expected behaviour (combined with the fact this is caught elsewhere), and therefore is probably a regression.

While we could use winfo class to determine the type of Tk widget, this would probably require a reasonably sized refactor of tkinter (and we would still need to support cases when it's a type we don't know). Therefore, I think returning None is the best solution.

Akuli, would you like to create a pull request for this?

[E-Paine]

Sorry, I should specify that we would use winfo class in order to then return a new tkinter object for the existing Tk widget (which currently cannot be done)

[serhiy-storchaka]

It is a duplicate of bpo-734176.

[Akuli]

I found bpo-734176 before I created this. It is NOT a duplicate. While bpo-734176 is about menus, this one is about focus_get(), and not necessarily related to menus. In fact, I initially noticed this with an "open file" dialog, not with a menu.

I'm not putting my address into your CLA, thank you very much.

[terryjreedy]

Akuli, what tk widgets do you think are not known to tkinter? In any case, tk menu is known to tkinter.

I cannot reproduce when running on Windows with 3.10.0b3: Add "print(root.children)" (after add_cascade) results in {'!menu': <tkinter.Menu object .!menu>, '!menu2': <tkinter.Menu object .!menu2>}. The names are created in tkinter.py lines 2564-2573.

I then see 'None' once and then '<tkinter.Tk object .>' indefinitely even while hovering over and clicking 'click me' and the dropdown. If I click outside the tk box, the print returns to 'None'.

Maybe there is an OS difference in what is considered to have 'focus'.

Key '#!menu' looks like '!menu' with '#' prepended. Someone could try changing the tkinter code referenced above and see if the change appears in the bad key. Also check the contents of root.children.

[terryjreedy]

I am not quite convinced that this is a duplicate of bpo-734176. The latter is about tearoff clones and nothing is cloned here. But I do notice that number 'names were also prefixed with '#'. What happens if 'tearoff=0' is added to the cascade so that it is not even clonable. The tkinter naming of instances after the class was added less than 10 years ago.