diff --git a/Lib/idlelib/editor.py b/Lib/idlelib/editor.py
index 606de71a6add88..9ae8758f566ebd 100644
--- a/Lib/idlelib/editor.py
+++ b/Lib/idlelib/editor.py
@@ -815,11 +815,11 @@ def ApplyKeybindings(self):
                 self.apply_bindings(xkeydefs)
         #update menu accelerators
         menuEventDict = {}
-        for menu in self.mainmenu.menudefs:
-            menuEventDict[menu[0]] = {}
-            for item in menu[1]:
+        for menu, items in self.mainmenu.menudefs.items():
+            menuEventDict[menu] = {}
+            for item in items:
                 if item:
-                    menuEventDict[menu[0]][prepstr(item[0])[1]] = item[1]
+                    menuEventDict[menu][prepstr(item[0])[1]] = item[1]
         for menubarItem in self.menudict:
             menu = self.menudict[menubarItem]
             end = menu.index(END)
@@ -1104,17 +1104,47 @@ def apply_bindings(self, keydefs=None):
                 text.event_add(event, *keylist)
 
     def fill_menus(self, menudefs=None, keydefs=None):
-        """Add appropriate entries to the menus and submenus
-
-        Menus that are absent or None in self.menudict are ignored.
+        """Add appropriate entries to the menus and submenus.
+
+        The default menudefs and keydefs are loaded from idlelib.mainmenu.
+        Menus that are absent or None in self.menudict are ignored.  The
+        default menu type created for submenus from menudefs is `command`.
+        A submenu item of None results in a `separator` menu type.
+        A submenu name beginning with ! represents a `checkbutton` type.
+
+        The menus are stored in self.menudict.
+
+        Args:
+            menudefs: Menu and submenu names, underlines (shortcuts),
+                and events which is a dictionary of the form:
+                {menu1: [(submenu1a, '<<virtual event>>'),
+                          (submenu1b, '<<virtual event>>'), ...],
+                 menu2: [(submenu2a, '<<virtual event>>'),
+                          (submenu2b, '<<virtual event>>'), ...],
+                }
+                Alternate format (may have been used in extensions):
+                [(menu1, [(submenu1a, '<<virtual event>>'),
+                          (submenu1b, '<<virtual event>>'), ...]),
+                 (menu2, [(submenu2a, '<<virtual event>>'),
+                          (submenu2b, '<<virtual event>>'), ...]),
+                ]
+            keydefs: Virtual events and keybinding definitions.  Used for
+                the 'accelerator' text on the menu.  Stored as a
+                dictionary of
+                {'<<virtual event>>': ['<binding1>', '<binding2>'],}
         """
         if menudefs is None:
             menudefs = self.mainmenu.menudefs
+        # menudefs was changed from a list of tuples to a dictionary.
+        # This conversion is needed for backward-compatibility for
+        # existing extensions that use the list format.
+        if isinstance(menudefs, list):
+            menudefs = dict(menudefs)
         if keydefs is None:
             keydefs = self.mainmenu.default_keydefs
         menudict = self.menudict
         text = self.text
-        for mname, entrylist in menudefs:
+        for mname, entrylist in menudefs.items():
             menu = menudict.get(mname)
             if not menu:
                 continue
diff --git a/Lib/idlelib/idle_test/test_editor.py b/Lib/idlelib/idle_test/test_editor.py
index 12bc8473668334..d203b111f97e64 100644
--- a/Lib/idlelib/idle_test/test_editor.py
+++ b/Lib/idlelib/idle_test/test_editor.py
@@ -2,8 +2,10 @@
 
 from idlelib import editor
 import unittest
+from unittest import mock
 from test.support import requires
-from tkinter import Tk
+import tkinter as tk
+from functools import partial
 
 Editor = editor.EditorWindow
 
@@ -13,7 +15,7 @@ class EditorWindowTest(unittest.TestCase):
     @classmethod
     def setUpClass(cls):
         requires('gui')
-        cls.root = Tk()
+        cls.root = tk.Tk()
         cls.root.withdraw()
 
     @classmethod
@@ -42,5 +44,106 @@ class dummy():
             self.assertEqual(func(dummy, inp), out)
 
 
+class MenubarTest(unittest.TestCase):
+    """Test functions involved with creating the menubar."""
+
+    @classmethod
+    def setUpClass(cls):
+        requires('gui')
+        root = cls.root = tk.Tk()
+        cls.root.withdraw()
+        # Test the functions called during the __init__ for
+        # EditorWindow that create the menubar and submenus.
+        # The class is mocked in order to prevent the functions
+        # from being called automatically.
+        w = cls.mock_editwin = mock.Mock(editor.EditorWindow)
+        w.menubar = tk.Menu(root, tearoff=False)
+        w.text = tk.Text(root)
+        w.tkinter_vars = {}
+
+    @classmethod
+    def tearDownClass(cls):
+        w = cls.mock_editwin
+        w.text.destroy()
+        w.menubar.destroy()
+        del w.menubar, w.text, w
+        cls.root.update_idletasks()
+        for id in cls.root.tk.call('after', 'info'):
+            cls.root.after_cancel(id)
+        cls.root.destroy()
+        del cls.root
+
+    def test_fill_menus(self):
+        eq = self.assertEqual
+        ed = editor.EditorWindow
+        w = self.mock_editwin
+        # Call real functions instead of mock.
+        fm = partial(editor.EditorWindow.fill_menus, w)
+        w.get_var_obj = ed.get_var_obj.__get__(w)
+
+        # Initialize top level menubar.
+        w.menudict = {}
+        edit = w.menudict['edit'] = tk.Menu(w.menubar, name='edit', tearoff=False)
+        win = w.menudict['windows'] = tk.Menu(w.menubar, name='windows', tearoff=False)
+        form = w.menudict['format'] = tk.Menu(w.menubar, name='format', tearoff=False)
+
+        # Submenus.
+        dict_menudefs = {'edit': [('_New', '<<open-new>>'),
+                                  None,
+                                  ('!Deb_ug', '<<debug>>')],
+                         'shell': [('_View', '<<view-restart>>'), ],
+                         'windows': [('Zoom Height', '<<zoom-height>>')],
+                        }
+        list_menudefs = [('edit', [('_New', '<<open-new>>'),
+                                  None,
+                                  ('!Deb_ug', '<<debug>>')]),
+                         ('shell', [('_View', '<<view-restart>>'), ]),
+                         ('windows', [('Zoom Height', '<<zoom-height>>')]),
+                        ]
+        keydefs = {'<<zoom-height>>': ['<Alt-Key-9>']}
+        for menudefs in (dict_menudefs, list_menudefs):
+            with self.subTest(menudefs=menudefs):
+                fm(menudefs, keydefs)
+
+                eq(edit.index('end'), 2)
+                eq(edit.type(0), tk.COMMAND)
+                eq(edit.entrycget(0, 'label'), 'New')
+                eq(edit.entrycget(0, 'underline'), 0)
+                self.assertIsNotNone(edit.entrycget(0, 'command'))
+                with self.assertRaises(tk.TclError):
+                    self.assertIsNone(edit.entrycget(0, 'var'))
+
+                eq(edit.type(1), tk.SEPARATOR)
+                with self.assertRaises(tk.TclError):
+                    self.assertIsNone(edit.entrycget(1, 'label'))
+
+                eq(edit.type(2), tk.CHECKBUTTON)
+                # Strip !.
+                eq(edit.entrycget(2, 'label'), 'Debug')
+                # Check that underline ignores !.
+                eq(edit.entrycget(2, 'underline'), 3)
+                self.assertIsNotNone(edit.entrycget(2, 'var'))
+                self.assertIn('<<debug>>', w.tkinter_vars)
+
+                eq(win.index('end'), 0)
+                eq(win.entrycget(0, 'underline'), -1)
+                eq(win.entrycget(0, 'accelerator'), 'Alt+9')
+
+                eq(form.index('end'), None)
+                self.assertNotIn('shell', w.menudict)
+
+                # Cleanup menus by deleting all menu items.
+                edit.delete(0, 2)
+                win.delete(0)
+                form.delete(0)
+
+        # Test defaults.
+        w.mainmenu.menudefs = ed.mainmenu.menudefs
+        w.mainmenu.default_keydefs = ed.mainmenu.default_keydefs
+        fm()
+        eq(form.index('end'), 9)  # Default Format menu has 10 items.
+        self.assertNotIn('run', w.menudict)
+
+
 if __name__ == '__main__':
     unittest.main(verbosity=2)
diff --git a/Lib/idlelib/idle_test/test_mainmenu.py b/Lib/idlelib/idle_test/test_mainmenu.py
index 7ec0368371c7df..147355d51a1868 100644
--- a/Lib/idlelib/idle_test/test_mainmenu.py
+++ b/Lib/idlelib/idle_test/test_mainmenu.py
@@ -8,7 +8,7 @@
 class MainMenuTest(unittest.TestCase):
 
     def test_menudefs(self):
-        actual = [item[0] for item in mainmenu.menudefs]
+        actual = list(mainmenu.menudefs.keys())
         expect = ['file', 'edit', 'format', 'run', 'shell',
                   'debug', 'options', 'window', 'help']
         self.assertEqual(actual, expect)
diff --git a/Lib/idlelib/macosx.py b/Lib/idlelib/macosx.py
index eeaab59ae80295..7894f2e5b55cc2 100644
--- a/Lib/idlelib/macosx.py
+++ b/Lib/idlelib/macosx.py
@@ -165,20 +165,21 @@ def overrideRootMenu(root, flist):
     from idlelib import mainmenu
     from idlelib import window
 
-    closeItem = mainmenu.menudefs[0][1][-2]
+    closeItem = mainmenu.menudefs['file'][-2]
 
     # Remove the last 3 items of the file menu: a separator, close window and
     # quit. Close window will be reinserted just above the save item, where
     # it should be according to the HIG. Quit is in the application menu.
-    del mainmenu.menudefs[0][1][-3:]
-    mainmenu.menudefs[0][1].insert(6, closeItem)
+    del mainmenu.menudefs['file'][-3:]
+    mainmenu.menudefs['file'].insert(6, closeItem)
 
     # Remove the 'About' entry from the help menu, it is in the application
     # menu
-    del mainmenu.menudefs[-1][1][0:2]
+    del mainmenu.menudefs['help'][0:2]
     # Remove the 'Configure Idle' entry from the options menu, it is in the
     # application menu as 'Preferences'
-    del mainmenu.menudefs[-3][1][0:2]
+    del mainmenu.menudefs['options'][0:2]
+
     menubar = Menu(root)
     root.configure(menu=menubar)
     menudict = {}
@@ -236,18 +237,18 @@ def help_dialog(event=None):
         menudict['application'] = menu = Menu(menubar, name='apple',
                                               tearoff=0)
         menubar.add_cascade(label='IDLE', menu=menu)
-        mainmenu.menudefs.insert(0,
-            ('application', [
-                ('About IDLE', '<<about-idle>>'),
-                    None,
-                ]))
+        appmenu = {'application': [
+                        ('About IDLE', '<<about-idle>>'),
+                         None,
+                        ]}
+        mainmenu.menudefs = {**appmenu, **mainmenu.menudefs}
     if isCocoaTk():
         # replace default About dialog with About IDLE one
         root.createcommand('tkAboutDialog', about_dialog)
         # replace default "Help" item in Help menu
         root.createcommand('::tk::mac::ShowHelp', help_dialog)
         # remove redundant "IDLE Help" from menu
-        del mainmenu.menudefs[-1][1][0]
+        del mainmenu.menudefs['help'][0]
 
 def fixb2context(root):
     '''Removed bad AquaTk Button-2 (right) and Paste bindings.
diff --git a/Lib/idlelib/mainmenu.py b/Lib/idlelib/mainmenu.py
index 1b8dc475650d8d..96c4bcd4c9ae71 100644
--- a/Lib/idlelib/mainmenu.py
+++ b/Lib/idlelib/mainmenu.py
@@ -19,9 +19,9 @@
 #   without altering overrideRootMenu() as well.
 #       TODO: Make this more robust
 
-menudefs = [
+menudefs = {
  # underscore prefixes character to underscore
- ('file', [
+ 'file': [
    ('_New File', '<<open-new-window>>'),
    ('_Open...', '<<open-window-from-file>>'),
    ('Open _Module...', '<<open-module>>'),
@@ -36,9 +36,9 @@
    None,
    ('_Close', '<<close-window>>'),
    ('E_xit', '<<close-all-windows>>'),
-   ]),
+   ],
 
- ('edit', [
+ 'edit': [
    ('_Undo', '<<undo>>'),
    ('_Redo', '<<redo>>'),
    None,
@@ -57,9 +57,9 @@
    ('E_xpand Word', '<<expand-word>>'),
    ('Show C_all Tip', '<<force-open-calltip>>'),
    ('Show Surrounding P_arens', '<<flash-paren>>'),
-   ]),
+   ],
 
- ('format', [
+ 'format': [
    ('_Indent Region', '<<indent-region>>'),
    ('_Dedent Region', '<<dedent-region>>'),
    ('Comment _Out Region', '<<comment-region>>'),
@@ -70,16 +70,16 @@
    ('New Indent Width', '<<change-indentwidth>>'),
    ('F_ormat Paragraph', '<<format-paragraph>>'),
    ('S_trip Trailing Whitespace', '<<do-rstrip>>'),
-   ]),
+   ],
 
- ('run', [
+ 'run': [
    ('Python Shell', '<<open-python-shell>>'),
    ('C_heck Module', '<<check-module>>'),
    ('R_un Module', '<<run-module>>'),
    ('Run... _Customized', '<<run-custom>>'),
    ]),
 
- ('shell', [
+ 'shell': [
    ('_View Last Restart', '<<view-restart>>'),
    ('_Restart Shell', '<<restart-shell>>'),
    None,
@@ -87,35 +87,35 @@
    ('_Next History', '<<history-next>>'),
    None,
    ('_Interrupt Execution', '<<interrupt-execution>>'),
-   ]),
+   ],
 
- ('debug', [
+ 'debug': [
    ('_Go to File/Line', '<<goto-file-line>>'),
    ('!_Debugger', '<<toggle-debugger>>'),
    ('_Stack Viewer', '<<open-stack-viewer>>'),
    ('!_Auto-open Stack Viewer', '<<toggle-jit-stack-viewer>>'),
-   ]),
+   ],
 
- ('options', [
+ 'options': [
    ('Configure _IDLE', '<<open-config-dialog>>'),
    None,
    ('Show _Code Context', '<<toggle-code-context>>'),
    ('Zoom Height', '<<zoom-height>>'),
-   ]),
+   ],
 
- ('window', [
-   ]),
+ 'window': [
+   ],
 
- ('help', [
+ 'help': [
    ('_About IDLE', '<<about-idle>>'),
    None,
    ('_IDLE Help', '<<help>>'),
    ('Python _Docs', '<<python-docs>>'),
-   ]),
-]
+   ],
+}
 
 if find_spec('turtledemo'):
-    menudefs[-1][1].append(('Turtle Demo', '<<open-turtle-demo>>'))
+    menudefs['help'].append(('Turtle Demo', '<<open-turtle-demo>>'))
 
 default_keydefs = idleConf.GetCurrentKeySet()
 
diff --git a/Lib/idlelib/zzdummy.py b/Lib/idlelib/zzdummy.py
index 8084499646653d..5d4aa553195fe6 100644
--- a/Lib/idlelib/zzdummy.py
+++ b/Lib/idlelib/zzdummy.py
@@ -7,12 +7,12 @@
 
 class ZzDummy:
 
-##    menudefs = [
-##        ('format', [
+##    menudefs = {
+##        'format': [
 ##            ('Z in', '<<z-in>>'),
 ##            ('Z out', '<<z-out>>'),
-##        ] )
-##    ]
+##        ]
+##    }
 
     def __init__(self, editwin):
         self.text = editwin.text

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