gh-138641: prevent accessing non-existing curframe in pdb commands

Lib/pdb.py

@@ -2647,6 +2647,16 @@ def set_trace(*, header=None, commands=None):
     just before debugging begins. *commands* is an optional list of
     pdb commands to run when the debugger starts.
     """
+    # Check if we're already in a pdb session by examining the call stack
+    frame = sys._getframe()
+    while frame:
+        if frame.f_code.co_name == 'interaction' and frame.f_code.co_filename.endswith('pdb.py'):
+            # We're already in a pdb session, just print a message and return
+            print("*** Nested breakpoint calls are not supported. "
+                  "Already running in the debugger.", file=sys.stderr)
+            return
+        frame = frame.f_back
+
     if Pdb._last_pdb_instance is not None:
         pdb = Pdb._last_pdb_instance
     else:
@@ -2662,6 +2672,16 @@ async def set_trace_async(*, header=None, commands=None):
     if they enter the debugger with this function. Otherwise it's the same
     as set_trace().
     """
+    # Check if we're already in a pdb session by examining the call stack
+    frame = sys._getframe()
+    while frame:
+        if frame.f_code.co_name == 'interaction' and frame.f_code.co_filename.endswith('pdb.py'):
+            # We're already in a pdb session, just print a message and return
+            print("*** Nested breakpoint calls are not supported. "
+                  "Already running in the debugger.", file=sys.stderr)
+            return
+        frame = frame.f_back
+
     if Pdb._last_pdb_instance is not None:
         pdb = Pdb._last_pdb_instance
     else:

Misc/NEWS.d/next/Library/2025-09-08-17-50-33.gh-issue-138641.nBPvKe.rst

@@ -0,0 +1,2 @@
+Prevent error when calling :func:`breakpoint` inside a :mod:`pdb` debugging session.
+Nested breakpoint calls now display a helpful error message instead of causing error.