[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, 2 insertions, 7 deletions

diff --git a/Lib/tkinter/tix.py b/Lib/tkinter/tix.py
index ac54550..ef1e740 100644
--- a/Lib/tkinter/tix.py
+++ b/Lib/tkinter/tix.py
@@ -387,9 +387,7 @@ class TixWidget(tkinter.Widget):
     # These are missing from Tkinter
     def image_create(self, imgtype, cnf={}, master=None, **kw):
         if not master:
-            master = tkinter._default_root
-            if not master:
-                raise RuntimeError('Too early to create image')
+            master = self
         if kw and cnf: cnf = _cnfmerge((cnf, kw))
         elif kw: cnf = kw
         options = ()
@@ -475,10 +473,7 @@ class DisplayStyle:
             elif 'refwindow' in cnf:
                 master = cnf['refwindow']
             else:
-                master = tkinter._default_root
-                if not master:
-                    raise RuntimeError("Too early to create display style: "
-                                       "no root window")
+                master = tkinter._get_default_root('create display style')
         self.tk = master.tk
         self.stylename = self.tk.call('tixDisplayStyle', itemtype,
                             *self._options(cnf,kw) )