diff -r 6216fb8afa53 Lib/idlelib/idle_test/htest.py
--- a/Lib/idlelib/idle_test/htest.py	Mon Jun 06 13:00:03 2016 +0300
+++ b/Lib/idlelib/idle_test/htest.py	Mon Jun 06 17:58:39 2016 -0400
@@ -66,7 +66,7 @@
 '''
 
 from importlib import import_module
-from idlelib.macosx import _initializeTkVariantTests
+from idlelib.macosx import _init_tk_type
 import tkinter as tk
 
 AboutDialog_spec = {
@@ -337,7 +337,7 @@
     root = tk.Tk()
     root.title('IDLE htest')
     root.resizable(0, 0)
-    _initializeTkVariantTests(root)
+    _init_tk_type(root)
 
     # a scrollable Label like constant width text widget.
     frameLabel = tk.Frame(root, padx=10)
diff -r 6216fb8afa53 Lib/idlelib/idle_test/test_configdialog.py
--- a/Lib/idlelib/idle_test/test_configdialog.py	Mon Jun 06 13:00:03 2016 +0300
+++ b/Lib/idlelib/idle_test/test_configdialog.py	Mon Jun 06 17:58:39 2016 -0400
@@ -8,14 +8,12 @@
 requires('gui')
 from tkinter import Tk
 import unittest
-from idlelib import macosx
 
 class ConfigDialogTest(unittest.TestCase):
 
     @classmethod
     def setUpClass(cls):
         cls.root = Tk()
-        macosx._initializeTkVariantTests(cls.root)
 
     @classmethod
     def tearDownClass(cls):
diff -r 6216fb8afa53 Lib/idlelib/idle_test/test_macosx.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/idlelib/idle_test/test_macosx.py	Mon Jun 06 17:58:39 2016 -0400
@@ -0,0 +1,69 @@
+'''Test idlelib.macosx.py
+'''
+from idlelib import macosx
+from test.support import requires
+import sys
+import tkinter as tk
+import unittest
+import unittest.mock as mock
+
+MAC = sys.platform == 'darwin'
+mactypes = {'carbon', 'cocoa', 'xquartz'}
+nontypes = {'other'}
+alltypes = mactypes | nontypes
+
+
+class InitTktypeTest(unittest.TestCase):
+    "Test _init_tk_type."
+
+    @classmethod
+    def setUpClass(cls):
+        requires('gui')
+        cls.root = tk.Tk()
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.root.update_idletasks()
+        cls.root.destroy()
+        del cls.root
+
+    def test_init_sets_tktype(self):
+        "Test that _init_tk_type sets _tk_type according to platform."
+        for root in (None, self.root):
+            with self.subTest(root=root):
+                macosx._tk_type == None
+                macosx._init_tk_type(root)
+                self.assertIn(macosx._tk_type,
+                              mactypes if MAC else nontypes)
+
+
+class IsTypeTkTest(unittest.TestCase):
+    "Test each of the four isTypeTk predecates."
+    isfuncs = ((macosx.isAquaTk, ('carbon', 'cocoa')),
+               (macosx.isCarbonTk, ('carbon')),
+               (macosx.isCocoaTk, ('cocoa')),
+               (macosx.isXQuartz, ('xquartz')),
+               )
+
+    @mock.patch('idlelib.macosx._init_tk_type')
+    def test_is_calls_init(self, mockinit):
+        "Test that each isTypeTk calls _init_tk_type when _tk_type is None."
+        macosx._tk_type = None
+        for func, whentrue in self.isfuncs:
+            with self.subTest(func=func):
+                func()
+                self.assertTrue(mockinit.called)
+                mockinit.reset_mock()
+
+    def test_isfuncs(self):
+        "Test that each isTypeTk return correct bool."
+        for func, whentrue in self.isfuncs:
+            for tktype in alltypes:
+                with self.subTest(func=func, whentrue=whentrue, tktype=tktype):
+                    macosx._tk_type = tktype
+                    (self.assertTrue if tktype in whentrue else self.assertFalse)\
+                                     (func())
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)
diff -r 6216fb8afa53 Lib/idlelib/macosx.py
--- a/Lib/idlelib/macosx.py	Mon Jun 06 13:00:03 2016 +0300
+++ b/Lib/idlelib/macosx.py	Mon Jun 06 17:58:39 2016 -0400
@@ -7,13 +7,14 @@
 
 _tk_type = None
 
-def _initializeTkVariantTests(root):
+def _init_tk_type(idleroot=None):
     """
     Initializes OS X Tk variant values for
     isAquaTk(), isCarbonTk(), isCocoaTk(), and isXQuartz().
     """
     global _tk_type
     if sys.platform == 'darwin':
+        root = idleroot or tkinter.Tk()
         ws = root.tk.call('tk', 'windowingsystem')
         if 'x11' in ws:
             _tk_type = "xquartz"
@@ -23,6 +24,8 @@
             _tk_type = "cocoa"
         else:
             _tk_type = "carbon"
+        if not idleroot:
+            root.destroy
     else:
         _tk_type = "other"
 
@@ -30,7 +33,8 @@
     """
     Returns True if IDLE is using a native OS X Tk (Cocoa or Carbon).
     """
-    assert _tk_type is not None
+    if not _tk_type:
+        _init_tk_type()
     return _tk_type == "cocoa" or _tk_type == "carbon"
 
 def isCarbonTk():
@@ -38,21 +42,24 @@
     Returns True if IDLE is using a Carbon Aqua Tk (instead of the
     newer Cocoa Aqua Tk).
     """
-    assert _tk_type is not None
+    if not _tk_type:
+        _init_tk_type()
     return _tk_type == "carbon"
 
 def isCocoaTk():
     """
     Returns True if IDLE is using a Cocoa Aqua Tk.
     """
-    assert _tk_type is not None
+    if not _tk_type:
+        _init_tk_type()
     return _tk_type == "cocoa"
 
 def isXQuartz():
     """
     Returns True if IDLE is using an OS X X11 Tk.
     """
-    assert _tk_type is not None
+    if not _tk_type:
+        _init_tk_type()
     return _tk_type == "xquartz"
 
 def tkVersionWarning(root):
@@ -222,7 +229,7 @@
     isAquaTk(), isCarbonTk(), isCocoaTk(), isXQuartz() functions which
     are initialized here as well.
     """
-    _initializeTkVariantTests(root)
+    _init_tk_type(root)
     if isAquaTk():
         hideTkConsole(root)
         overrideRootMenu(root, flist)