bpo-32857: Raise error when tkinter after_cancel() is called with None. (GH-5701)

(cherry picked from commit 74382a3f)
Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>

Lib/tkinter/__init__.py

@@ -739,6 +739,7 @@ class Misc:
         if not func:
             # I'd rather use time.sleep(ms*0.001)
             self.tk.call('after', ms)
+            return None
         else:
             def callit():
                 try:
@@ -762,11 +763,13 @@ class Misc:
         """Cancel scheduling of function identified with ID.
 
         Identifier returned by after or after_idle must be
-        given as first parameter."""
+        given as first parameter.
+        """
+        if not id:
+            raise ValueError('id must be a valid identifier returned from '
+                             'after or after_idle')
         try:
             data = self.tk.call('after', 'info', id)
-            # In Tk 8.3, splitlist returns: (script, type)
-            # In Tk 8.4, splitlist may return (script, type) or (script,)
             script = self.tk.splitlist(data)[0]
             self.deletecommand(script)
         except TclError:

Lib/tkinter/test/test_tkinter/test_misc.py

@@ -48,6 +48,114 @@ class MiscTest(AbstractTkTest, unittest.TestCase):
                 '^must specify a background color$',
                 root.tk_setPalette, highlightColor='blue')
 
+    def test_after(self):
+        root = self.root
+
+        def callback(start=0, step=1):
+            nonlocal count
+            count = start + step
+
+        # Without function, sleeps for ms.
+        self.assertIsNone(root.after(1))
+
+        # Set up with callback with no args.
+        count = 0
+        timer1 = root.after(0, callback)
+        self.assertIn(timer1, root.tk.call('after', 'info'))
+        (script, _) = root.tk.splitlist(root.tk.call('after', 'info', timer1))
+        root.update()  # Process all pending events.
+        self.assertEqual(count, 1)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call(script)
+
+        # Set up with callback with args.
+        count = 0
+        timer1 = root.after(0, callback, 42, 11)
+        root.update()  # Process all pending events.
+        self.assertEqual(count, 53)
+
+        # Cancel before called.
+        timer1 = root.after(1000, callback)
+        self.assertIn(timer1, root.tk.call('after', 'info'))
+        (script, _) = root.tk.splitlist(root.tk.call('after', 'info', timer1))
+        root.after_cancel(timer1)  # Cancel this event.
+        self.assertEqual(count, 53)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call(script)
+
+    def test_after_idle(self):
+        root = self.root
+
+        def callback(start=0, step=1):
+            nonlocal count
+            count = start + step
+
+        # Set up with callback with no args.
+        count = 0
+        idle1 = root.after_idle(callback)
+        self.assertIn(idle1, root.tk.call('after', 'info'))
+        (script, _) = root.tk.splitlist(root.tk.call('after', 'info', idle1))
+        root.update_idletasks()  # Process all pending events.
+        self.assertEqual(count, 1)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call(script)
+
+        # Set up with callback with args.
+        count = 0
+        idle1 = root.after_idle(callback, 42, 11)
+        root.update_idletasks()  # Process all pending events.
+        self.assertEqual(count, 53)
+
+        # Cancel before called.
+        idle1 = root.after_idle(callback)
+        self.assertIn(idle1, root.tk.call('after', 'info'))
+        (script, _) = root.tk.splitlist(root.tk.call('after', 'info', idle1))
+        root.after_cancel(idle1)  # Cancel this event.
+        self.assertEqual(count, 53)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call(script)
+
+    def test_after_cancel(self):
+        root = self.root
+
+        def callback():
+            nonlocal count
+            count += 1
+
+        timer1 = root.after(5000, callback)
+        idle1 = root.after_idle(callback)
+
+        # No value for id raises a ValueError.
+        with self.assertRaises(ValueError):
+            root.after_cancel(None)
+
+        # Cancel timer event.
+        count = 0
+        (script, _) = root.tk.splitlist(root.tk.call('after', 'info', timer1))
+        root.tk.call(script)
+        self.assertEqual(count, 1)
+        root.after_cancel(timer1)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call(script)
+        self.assertEqual(count, 1)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call('after', 'info', timer1)
+
+        # Cancel same event - nothing happens.
+        root.after_cancel(timer1)
+
+        # Cancel idle event.
+        count = 0
+        (script, _) = root.tk.splitlist(root.tk.call('after', 'info', idle1))
+        root.tk.call(script)
+        self.assertEqual(count, 1)
+        root.after_cancel(idle1)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call(script)
+        self.assertEqual(count, 1)
+        with self.assertRaises(tkinter.TclError):
+            root.tk.call('after', 'info', idle1)
+
 
 tests_gui = (MiscTest, )
 

Misc/NEWS.d/next/Library/2018-02-16-14-37-14.bpo-32857.-XljAx.rst

+In :mod:`tkinter`, ``after_cancel(None)`` now raises a :exc:`ValueError` instead of canceling the first scheduled function.  Patch by Cheryl Sabella.