[3.8] bpo-42630: Improve error reporting in Tkinter for absent default root (GH-23781) (GH-23854)

* Tkinter functions and constructors which need a default root window
  raise now RuntimeError with descriptive message instead of obscure
  AttributeError or NameError if it is not created yet or cannot
  be created automatically.

* Add tests for all functions which use default root window.

* Fix import in the pynche script.

(cherry picked from commit 3d569fd6dccf9f582bafaca04d3535094cae393e)

1 files changed, 4 insertions, 8 deletions

diff --git a/Tools/pynche/PyncheWidget.py b/Tools/pynche/PyncheWidget.py
index 364f22b..59b4137 100644
--- a/Tools/pynche/PyncheWidget.py
+++ b/Tools/pynche/PyncheWidget.py
@@ -36,15 +36,11 @@ class PyncheWidget:
         else:
             # Is there already a default root for Tk, say because we're
             # running under Guido's IDE? :-) Two conditions say no, either the
-            # import fails or _default_root is None.
-            tkroot = None
-            try:
-                from Tkinter import _default_root
-                tkroot = self.__tkroot = _default_root
-            except ImportError:
-                pass
+            # _default_root is None or it is unset.
+            tkroot = getattr(tkinter, '_default_root', None)
             if not tkroot:
-                tkroot = self.__tkroot = Tk(className='Pynche')
+                tkroot = Tk(className='Pynche')
+            self.__tkroot = tkroot
             # but this isn't our top level widget, so make it invisible
             tkroot.withdraw()
         # create the menubar