gh-55646: Do not crash IDLE on an invalid key binding

A typo in a key binding, such as <Alt-Key-up> for <Alt-Key-Up>, crashed IDLE
at startup.  It is now ignored with a warning.

Author: serhiy-storchaka

Lib/idlelib/idle_test/test_multicall.py

@@ -1,9 +1,11 @@
 "Test multicall, coverage 33%."
 
 from idlelib import multicall
+import io
 import unittest
 from test.support import requires
 from tkinter import Tk, Text
+from unittest import mock
 
 
 class MultiCallTest(unittest.TestCase):
@@ -43,6 +45,15 @@ def test_yview(self):
         mctext = self.mc(self.root)
         self.assertIs(mctext.yview.__func__, Text.yview)
 
+    def test_invalid_binding(self):
+        # gh-55646: an invalid key binding must not crash IDLE.
+        mctext = self.mc(self.root)
+        # 'up' is not a keysym; it should be 'Up'.
+        mctext.event_add('<<test-bad>>', '<Alt-Key-up>')
+        with mock.patch('sys.stderr', new_callable=io.StringIO) as stderr:
+            mctext.bind('<<test-bad>>', lambda e: None)  # Must not raise.
+        self.assertIn('invalid key binding', stderr.getvalue())
+
 
 if __name__ == '__main__':
     unittest.main(verbosity=2)

Lib/idlelib/multicall.py

@@ -310,6 +310,14 @@ def _triplet_to_sequence(triplet):
     else:
         return '<'+_state_names[triplet[0]]+_types[triplet[1]][0]+'>'
 
+
+def _warn_bad_binding(triplet, err):
+    # Ignore a key binding invalidated by a typo in the user's config
+    # instead of crashing IDLE (gh-55646).
+    print(f'Warning: ignoring invalid key binding '
+          f'{_triplet_to_sequence(triplet)!r}: {err}', file=sys.stderr)
+
+
 _multicall_dict = {}
 def MultiCallCreator(widget):
     """Return a MultiCall class which inherits its methods from the
@@ -343,8 +351,15 @@ def bind(self, sequence=None, func=None, add=None):
                             self.__binders[triplet[1]].unbind(triplet, ei[0])
                     ei[0] = func
                     if ei[0] is not None:
+                        bad = []
                         for triplet in ei[1]:
-                            self.__binders[triplet[1]].bind(triplet, func)
+                            try:
+                                self.__binders[triplet[1]].bind(triplet, func)
+                            except tkinter.TclError as err:
+                                _warn_bad_binding(triplet, err)
+                                bad.append(triplet)
+                        for triplet in bad:  # Drop the invalid sequences.
+                            ei[1].remove(triplet)
                 else:
                     self.__eventinfo[sequence] = [func, []]
             return widget.bind(self, sequence, func, add)
@@ -374,7 +389,11 @@ def event_add(self, virtual, *sequences):
                     widget.event_add(self, virtual, seq)
                 else:
                     if func is not None:
-                        self.__binders[triplet[1]].bind(triplet, func)
+                        try:
+                            self.__binders[triplet[1]].bind(triplet, func)
+                        except tkinter.TclError as err:
+                            _warn_bad_binding(triplet, err)
+                            continue  # Drop the invalid sequence.
                     triplets.append(triplet)
 
         def event_delete(self, virtual, *sequences):

Misc/NEWS.d/next/IDLE/2026-07-01-21-00-00.gh-issue-55646.kBind9.rst

@@ -0,0 +1,3 @@
+Fix IDLE crash at startup when the user configuration contains an invalid key
+binding, such as ``<Alt-Key-up>`` instead of ``<Alt-Key-Up>``.  The invalid
+binding is now ignored with a warning.