gh-160643: Reset Bdb.enterframe when trace dispatch raises

An exception escaping trace_dispatch(), such as BdbQuit after q in the
interactive interpreter, left enterframe set, so every later
breakpoint() returned early. Reset it in a finally block.

Author: tonghuaroot

Lib/bdb.py

@@ -251,8 +251,10 @@ def reset(self):
     @contextmanager
     def set_enterframe(self, frame):
         self.enterframe = frame
-        yield
-        self.enterframe = None
+        try:
+            yield
+        finally:
+            self.enterframe = None
 
     def trace_dispatch(self, frame, event, arg):
         """Dispatch a trace function for debugged frames based on the event.

Lib/test/test_pdb.py

@@ -5047,6 +5047,27 @@ def test_nested_set_trace(self):
         self.assertIn('42', stdout)
         self.assertEqual(stderr, '')
 
+    def test_set_trace_after_quit(self):
+        script = """
+            import bdb
+            import sys
+            # Make quit raise BdbQuit as in the interactive interpreter.
+            sys.ps1 = '>>> '
+            try:
+                breakpoint()
+            except bdb.BdbQuit:
+                pass
+            breakpoint()
+        """
+        commands = """
+            q
+            p 'stopped' + 'again'
+            c
+        """
+        stdout, stderr = self._run_script(script, commands)
+        self.assertIn("'stoppedagain'", stdout)
+        self.assertEqual(stderr, '')
+
     def test_readline_not_imported(self):
         """GH-138860
         Directly or indirectly importing readline might deadlock a subprocess

Misc/NEWS.d/next/Library/2026-10-24-17-46-57.gh-issue-160643.xOwxMs.rst

@@ -0,0 +1,3 @@
+Fix :func:`breakpoint` and :func:`pdb.set_trace` being ignored after a
+previous debugger session was ended by an exception, for example by quitting
+with ``q`` in the interactive interpreter.