tkinter.simpledialog: KeyError when a dialog is opened while a window not created by tkinter holds the grab

Bug report

simpledialog.SimpleDialog.go() and Dialog use _temp_grab_focus(), which calls grab_current() and focus_get() to restore the previous grab and focus afterwards. Both convert the Tcl window name with _nametowidget(), which raises KeyError for windows which were not created by tkinter, such as the native message box (.__tk__messagebox) or Tk's file dialogs on X11.

import tkinter
from tkinter import messagebox, simpledialog
root = tkinter.Tk()
root.after(300, lambda: simpledialog.SimpleDialog(root, text='hi', buttons=['OK']).go())
messagebox.showinfo('t', 'm', parent=root)

KeyError: '__tk__messagebox'

This happens in IDLE when a dialog is opened from an event delivered while a message box is shown (#153481). Regression from #151848 (3.16).

Linked PRs

• gh-157677

Start by reading tkinter.simpledialog.SimpleDialog.go(), Dialog, and _temp_grab_focus(), then run the supplied messagebox and simpledialog reproduction. The fix is complete when opening a dialog while a non-tk window holds the grab no longer raises KeyError and the previous grab and focus are restored.