Issue #18604: Consolidated checks for GUI availability.

test_support._is_gui_available is now defined the same way on every
platform, and now includes the Windows-specific check that had been in the
Windows version of _is_gui_available and the OSX-specific check that was
in tkinter.test.support.check_tk_availability.  Also, every platform
checks whether Tk can be instantiated (if the platform-specific checks
passed).

Lib/tkinter/test/support.py

@@ -1,52 +1,10 @@
 import sys
 import tkinter
 import unittest
-
-_tk_unavailable = None
-
-def check_tk_availability():
-    """Check that Tk is installed and available."""
-    global _tk_unavailable
-
-    if _tk_unavailable is None:
-        _tk_unavailable = False
-        if sys.platform == 'darwin':
-            # The Aqua Tk implementations on OS X can abort the process if
-            # being called in an environment where a window server connection
-            # cannot be made, for instance when invoked by a buildbot or ssh
-            # process not running under the same user id as the current console
-            # user.  To avoid that, raise an exception if the window manager
-            # connection is not available.
-            from ctypes import cdll, c_int, pointer, Structure
-            from ctypes.util import find_library
-
-            app_services = cdll.LoadLibrary(find_library("ApplicationServices"))
-
-            if app_services.CGMainDisplayID() == 0:
-                _tk_unavailable = "cannot run without OS X window manager"
-            else:
-                class ProcessSerialNumber(Structure):
-                    _fields_ = [("highLongOfPSN", c_int),
-                                ("lowLongOfPSN", c_int)]
-                psn = ProcessSerialNumber()
-                psn_p = pointer(psn)
-                if (  (app_services.GetCurrentProcess(psn_p) < 0) or
-                      (app_services.SetFrontProcess(psn_p) < 0) ):
-                    _tk_unavailable = "cannot run without OS X gui process"
-        else:   # not OS X
-            import tkinter
-            try:
-                tkinter.Button()
-            except tkinter.TclError as msg:
-                # assuming tk is not available
-                _tk_unavailable = "tk not available: %s" % msg
-
-    if _tk_unavailable:
-        raise unittest.SkipTest(_tk_unavailable)
-    return
+from test.support import requires
 
 def get_tk_root():
-    check_tk_availability()     # raise exception if tk unavailable
+    requires('gui')             # raise exception if tk unavailable
     try:
         root = tkinter._default_root
     except AttributeError: