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

* 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.

Lib/tkinter/__init__.py

@@ -270,7 +270,7 @@ class Event:
         )
 
 
-_support_default_root = 1
+_support_default_root = True
 _default_root = None
 
 
@@ -280,13 +280,26 @@ def NoDefaultRoot():
     Call this function to inhibit that the first instance of
     Tk is used for windows without an explicit parent window.
     """
-    global _support_default_root
-    _support_default_root = 0
-    global _default_root
+    global _support_default_root, _default_root
+    _support_default_root = False
+    # Delete, so any use of _default_root will immediately raise an exception.
+    # Rebind before deletion, so repeated calls will not fail.
     _default_root = None
     del _default_root
 
 
+def _get_default_root(what=None):
+    if not _support_default_root:
+        raise RuntimeError("No master specified and tkinter is "
+                           "configured to not support default root")
+    if not _default_root:
+        if what:
+            raise RuntimeError(f"Too early to {what}: no default root window")
+        root = Tk()
+        assert _default_root is root
+    return _default_root
+
+
 def _tkerror(err):
     """Internal function."""
     pass
@@ -330,7 +343,7 @@ class Variable:
             raise TypeError("name must be a string")
         global _varnum
         if not master:
-            master = _default_root
+            master = _get_default_root('create variable')
         self._root = master._root()
         self._tk = master.tk
         if name:
@@ -591,7 +604,7 @@ class BooleanVar(Variable):
 
 def mainloop(n=0):
     """Run the main loop of Tcl."""
-    _default_root.tk.mainloop(n)
+    _get_default_root('run the main loop').tk.mainloop(n)
 
 
 getint = int
@@ -600,9 +613,9 @@ getdouble = float
 
 
 def getboolean(s):
-    """Convert true and false to integer values 1 and 0."""
+    """Convert Tcl object to True or False."""
     try:
-        return _default_root.tk.getboolean(s)
+        return _get_default_root('use getboolean()').tk.getboolean(s)
     except TclError:
         raise ValueError("invalid literal for getboolean()")
 
@@ -2248,7 +2261,7 @@ class Tk(Misc, Wm):
         is the name of the widget class."""
         self.master = None
         self.children = {}
-        self._tkloaded = 0
+        self._tkloaded = False
         # to avoid recursions in the getattr code in case of failure, we
         # ensure that self.tk is always _something_.
         self.tk = None
@@ -2272,7 +2285,7 @@ class Tk(Misc, Wm):
             self._loadtk()
 
     def _loadtk(self):
-        self._tkloaded = 1
+        self._tkloaded = True
         global _default_root
         # Version sanity checks
         tk_version = self.tk.getvar('tk_version')
@@ -2521,12 +2534,8 @@ class BaseWidget(Misc):
 
     def _setup(self, master, cnf):
         """Internal function. Sets up information about children."""
-        if _support_default_root:
-            global _default_root
-            if not master:
-                if not _default_root:
-                    _default_root = Tk()
-                master = _default_root
+        if not master:
+            master = _get_default_root()
         self.master = master
         self.tk = master.tk
         name = None
@@ -3990,9 +3999,7 @@ class Image:
     def __init__(self, imgtype, name=None, cnf={}, master=None, **kw):
         self.name = None
         if not master:
-            master = _default_root
-            if not master:
-                raise RuntimeError('Too early to create image')
+            master = _get_default_root('create image')
         self.tk = getattr(master, 'tk', master)
         if not name:
             Image._last_id += 1
@@ -4146,11 +4153,13 @@ class BitmapImage(Image):
 
 
 def image_names():
-    return _default_root.tk.splitlist(_default_root.tk.call('image', 'names'))
+    tk = _get_default_root('use image_names()').tk
+    return tk.splitlist(tk.call('image', 'names'))
 
 
 def image_types():
-    return _default_root.tk.splitlist(_default_root.tk.call('image', 'types'))
+    tk = _get_default_root('use image_types()').tk
+    return tk.splitlist(tk.call('image', 'types'))
 
 
 class Spinbox(Widget, XView):