From 68d5c6e8ff77a0ac7df8d4cb1ac1fc5ccbd47f46 Mon Sep 17 00:00:00 2001
From: Tian Gao <gaogaotiantian@hotmail.com>
Date: Tue, 20 Oct 2026 15:47:31 -0700
Subject: [PATCH] gh-138641: Ignore set_trace() when we are already in pdb
 (GH-159918) (cherry picked from commit
 514cfe739e8559d964a216e20358ae6c0f8831fa)

Co-authored-by: Tian Gao <gaogaotiantian@hotmail.com>
---
 Lib/bdb.py                                           |  7 +++++++
 Lib/test/test_pdb.py                                 | 12 ++++++++++++
 .../2026-10-20-17-29-14.gh-issue-138641.FN6qM0.rst   |  1 +
 3 files changed, 20 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-20-17-29-14.gh-issue-138641.FN6qM0.rst

diff --git a/Lib/bdb.py b/Lib/bdb.py
index 932fe45ba7f587..da179e72eb3604 100644
--- a/Lib/bdb.py
+++ b/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
diff --git a/Lib/test/test_pdb.py b/Lib/test/test_pdb.py
index 12f33bf4f99855..bafbe2f086e85c 100644
--- a/Lib/test/test_pdb.py
+++ b/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
diff --git a/Misc/NEWS.d/next/Library/2026-10-20-17-29-14.gh-issue-138641.FN6qM0.rst b/Misc/NEWS.d/next/Library/2026-10-20-17-29-14.gh-issue-138641.FN6qM0.rst
new file mode 100644
index 00000000000000..6d9ac174282839
--- /dev/null
+++ b/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.