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

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.

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?

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)

It is a duplicate of issue734176.

I found issue734176 before I created this. It is NOT a duplicate. While issue734176 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.

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.

I am not quite convinced that this is a duplicate of #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.

Unfortunately I don't know any real-world examples of this on Windows. The open file dialog works very differently on Windows: it uses the native Windows dialog, whereas on Linux, it's implemented in Tcl.

Meanwhile, here's a platform-independent toy example:

    import tkinter

    root = tkinter.Tk()
    root.tk.eval("""
    entry .e
    pack .e
    focus .e
    """)
    root.after(500, root.focus_get)
    root.mainloop()

Also, thanks for reopening!

Traceback (most recent call last):
  File "C:\Programs\Python310\lib\tkinter\__init__.py", line 1921, in __call__
    return self.func(*args)
  File "C:\Programs\Python310\lib\tkinter\__init__.py", line 839, in callit
    func(*args)
  File "C:\Programs\Python310\lib\tkinter\__init__.py", line 783, in focus_get
    return self._nametowidget(name)
  File "C:\Programs\Python310\lib\tkinter\__init__.py", line 1536, in nametowidget
    w = w.children[n]
KeyError: 'e'

Is catching KeyError in the following
            try:
                # Tcl sometimes returns extra windows, e.g. for
                # menus; those need to be skipped
                result.append(self._nametowidget(child))
            except KeyError:
                pass
really correct?  It appears to skip things that *can* get focus by key or mouse action.  But what choice is there?

Silently failing when asked to focus on something is even less obviously correct.  For 'widget = root.focus_get' to assign None to widget is not obviously useful as it likely just delays the error.

Here are the options:

- Do nothing. My program will error in some corner cases.

- Change it to return `None`, so `widget.focus_get() is not None` no longer means "this application doesn't have focus", but rather "this application doesn't have focus or the focused widget was not created in tkinter". Note that `focus_get()` can already return None, and we would just add one more situation where it does so.

- Change tkinter so that it doesn't matter whether a widget was created in tkinter or not. This doesn't seem to be easy.

Typo in previous message: I meant `widget.focus_get() is None`. It currently means "this application doesn't have focus", while `is not None` currently means "this application has focus".