bpo-42721: Improve using simple dialogs without root window (GH-23897)

When simple query dialogs (tkinter.simpledialog), message boxes
(tkinter.messagebox) or color choose dialog (tkinter.colorchooser)
are created without arguments master and parent, and the default
root window is not yet created, a new temporary hidden root window
will be created automatically. It will not be set as the default root
window and will be destroyed right after closing the dialog window.
It will help to use these simple dialog windows in programs which do
not need other GUI.

Previously, message boxes and color chooser created the blank root
window and left it after closing the dialog window, and query dialogs
just raised an exception.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/tkinter/commondialog.py

@@ -10,7 +10,7 @@
 
 __all__ = ["Dialog"]
 
-from tkinter import Frame
+from tkinter import Frame, _get_temp_root, _destroy_temp_root
 
 
 class Dialog:
@@ -37,22 +37,17 @@ class Dialog:
 
         self._fixoptions()
 
-        # we need a dummy widget to properly process the options
-        # (at least as long as we use Tkinter 1.63)
-        w = Frame(self.master)
-
+        master = self.master
+        if master is None:
+            master = _get_temp_root()
         try:
-
-            s = w.tk.call(self.command, *w._options(self.options))
-
-            s = self._fixresult(w, s)
-
+            self._test_callback(master)  # The function below is replaced for some tests.
+            s = master.tk.call(self.command, *master._options(self.options))
+            s = self._fixresult(master, s)
         finally:
-
-            try:
-                # get rid of the widget
-                w.destroy()
-            except:
-                pass
+            _destroy_temp_root(master)
 
         return s
+
+    def _test_callback(self, master):
+        pass