Issue #20567: Delete class attribute gui widgets in idle tests.

Code patch by Serhiy Storchaka

Lib/idlelib/idle_test/README.txt

@@ -41,9 +41,10 @@ idle class. For the benefit of buildbot machines that do not have a graphics
 screen, gui tests must be 'guarded' by "requires('gui')" in a setUp
 function or method. This will typically be setUpClass.
 
-All gui objects must be destroyed by the end of the test, perhaps in a tearDown
-function. Creating the Tk root directly in a setUp allows a reference to be saved
-so it can be properly destroyed in the corresponding tearDown. 
+To avoid interfering with other gui tests, all gui objects must be destroyed
+and deleted by the end of the test.  If a widget, such as a Tk root, is created
+in a setUpX function, destroy it in the corresponding tearDownX.  For module
+and class attributes, also delete the widget.
 ---
     @classmethod
     def setUpClass(cls):
@@ -53,6 +54,7 @@ so it can be properly destroyed in the corresponding tearDown.
     @classmethod
     def tearDownClass(cls):
         cls.root.destroy()
+        del cls.root
 ---
 
 Support.requires('gui') returns true if it is either called in a main module

Lib/idlelib/idle_test/test_formatparagraph.py

@@ -277,6 +277,9 @@ class FormatEventTest(unittest.TestCase):
     @classmethod
     def tearDownClass(cls):
         cls.root.destroy()
+        del cls.root
+        del cls.text
+        del cls.formatter
 
     def test_short_line(self):
         self.text.insert('1.0', "Short line\n")

Lib/idlelib/idle_test/test_idlehistory.py

@@ -80,6 +80,7 @@ class FetchTest(unittest.TestCase):
     @classmethod
     def tearDownClass(cls):
         cls.root.destroy()
+        del cls.root
 
     def fetch_test(self, reverse, line, prefix, index, *, bell=False):
         # Perform one fetch as invoked by Alt-N or Alt-P

Lib/idlelib/idle_test/test_searchengine.py

@@ -64,6 +64,7 @@ class GetSelectionTest(unittest.TestCase):
 ##    @classmethod
 ##    def tearDownClass(cls):
 ##        cls.root.destroy()
+##        del cls.root
 
     def test_get_selection(self):
         # text = Text(master=self.root)
@@ -219,6 +220,7 @@ class SearchTest(unittest.TestCase):
 ##    @classmethod
 ##    def tearDownClass(cls):
 ##        cls.root.destroy()
+##        del cls.root
 
     def test_search(self):
         Equal = self.assertEqual
@@ -261,6 +263,7 @@ class ForwardBackwardTest(unittest.TestCase):
 ##    @classmethod
 ##    def tearDownClass(cls):
 ##        cls.root.destroy()
+##        del cls.root
 
     @classmethod
     def setUpClass(cls):

Lib/idlelib/idle_test/test_text.py

@@ -221,6 +221,7 @@ class TkTextTest(TextTest, unittest.TestCase):
     @classmethod
     def tearDownClass(cls):
         cls.root.destroy()
+        del cls.root
 
 
 if __name__ == '__main__':

Lib/test/test_idle.py

@@ -14,6 +14,7 @@ if use_resources and 'gui' in use_resources:
     try:
         root = tk.Tk()
         root.destroy()
+        del root
     except tk.TclError:
         while 'gui' in use_resources:
             use_resources.remove('gui')