`pdb` commands may try to access non-existing frames

[yihong0618]

Bug report

Bug description:

➜  cpython git:(main) ./python.exe
Python 3.15.0a0 (heads/main:919c7e806e, Sep  8 2025, 17:24:03) [Clang 17.0.0 (clang-1700.0.13.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> breakpoint()
> <python-input-0>(1)<module>()
(Pdb) breakpoint()
(Pdb) ll
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    breakpoint()
    ~~~~~~~~~~^^
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 94, in wrapper
    ret = func(frame, *args)
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 154, in opcode_callback
    frame.f_trace(frame, 'opcode', None)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 289, in trace_dispatch
    return self.dispatch_opcode(frame, arg)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 406, in dispatch_opcode
    self.user_opcode(frame)
    ~~~~~~~~~~~~~~~~^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 547, in user_line
    self.interaction(frame, None)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 736, in interaction
    self._cmdloop()
    ~~~~~~~~~~~~~^^
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 601, in _cmdloop
    self.cmdloop()
    ~~~~~~~~~~~~^^
  File "/Users/hyi/prs/cpython/Lib/cmd.py", line 149, in cmdloop
    stop = self.onecmd(line)
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 1038, in onecmd
    return cmd.Cmd.onecmd(self, line)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/cmd.py", line 227, in onecmd
    return func(arg)
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 2156, in do_longlist
    filename = self.curframe.f_code.co_filename
               ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'f_code'
>>> 

CPython versions tested on:

3.15, CPython main branch

Operating systems tested on:

Other

Linked PRs

• gh-138641: prevent accessing non-existing curframe in pdb commands #138642

[yihong0618]

I am not sure it needs to fix, so will draft a patch

[picnixz]

It's not a crash but it's an annoying situation as the error message is really not needed. So yes a fix is needed.

[gaogaotiantian]

I think we are fixing the wrong thing. So this behavior should not happen, but checking self.curframe is not the way to fix it. self.curframe should always exist. The thing we should fix is how we treat breakpoint() in a breakpoint.

[yihong0618]

I think we are fixing the wrong thing. So this behavior should not happen, but checking self.curframe is not the way to fix it. self.curframe should always exist. The thing we should fix is how we treat breakpoint() in a breakpoint.

copy that, will try to learn that way, thank you

[ChenyangLi4288]

Thank you! @gaogaotiantian I am thinking several ways to fix it. One is to make breakpoint() a no-op in pdb context - Detect when we're already in a debugger and just return immediately. Or simply don't trigger the opcode callback mechanism for the breakpoint() builtin when already debugging.

The other way is to ensure that nested debugger calls properly maintain frame context.

What do you think?