[3.14] gh-138641: Ignore set_trace() when we are already in pdb (GH-159918)

Lib/bdb.py

@@ -610,6 +610,13 @@ def set_trace(self, frame=None):
 
         If frame is not specified, debugging starts from caller's frame.
         """
+
+        if self.enterframe is not None:
+            # If self.enterframe is not None, it means we are already in
+            # dispatched functions. Tracing is disabled when calling a
+            # trace function, so we just skip this set_trace() call.
+            return
+
         self.stop_trace()
         if frame is None:
             frame = sys._getframe().f_back

Lib/test/test_pdb.py

@@ -4875,6 +4875,18 @@ def foo():
         stdout, _ = self._run_script(script, commands)
         self.assertIn("42", stdout)
 
+    def test_nested_set_trace(self):
+        # Calling breakpoint() in pdb prompt should just be ignored.
+        script = "breakpoint()"
+        commands = """
+            breakpoint()
+            p 40 + 2
+            c
+        """
+        stdout, stderr = self._run_script(script, commands)
+        self.assertIn('42', 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/2026-10-20-17-29-14.gh-issue-138641.FN6qM0.rst

@@ -0,0 +1 @@
+Ignore :func:`pdb.set_trace` when called directly at the :mod:`pdb` prompt.