New REPL sometimes incrementally indents traceback lines

[picnixz]

Bug report

Bug description:

The reproducer I found comes from #84376 and it was entirely by chance that I stumbled upon the REPL issue itself. I don't know if it's because we're using multiprocessing or if it's because the exception is in another thread or process, or if it's again something else.

Python 3.14.0a6+ (heads/main:3b3720f1a26, Apr  1 2025, 10:14:27) [GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import multiprocessing
... q = multiprocessing.Queue()
... try:
...     q.put(lambda: None)
...     print("Success!")
... except Exception:
...     print("Failure")
...
Success!
>>> AttributeError: module '__main__' has no attribute '<lambda>'

                                                                 During handling of the above exception, another exception occurred:

                                                                                                                                    Traceback (most recent call last):
                                                                                                                                                                        File "/$HOME/lib/python/cpython/Lib/multiprocessing/queues.py", line 262, in _feed
                                                                                           obj = _ForkingPickler.dumps(obj)
                                                                                                                             File "/$HOME/lib/python/cpython/Lib/multiprocessing/reduction.py", line 51, in dumps
                                                  cls(buf, protocol).dump(obj)
                                                                                  ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
                                                                                                              _pickle.PicklingError: Can't pickle <function <lambda> at 0x7f1410a13a10>: it's not found as __main__.<lambda>

KeyboardInterrupt
>>>

In the old REPL, the traceback is properly shown.

Python 3.14.0a6+ (heads/main:3b3720f1a26, Apr  1 2025, 10:14:27) [GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import multiprocessing
>>> q = multiprocessing.Queue()
>>> try:
...     q.put(lambda: None)
...     print("Success!")
... except Exception:
...     print("Failure")
...
Success!
>>> AttributeError: module '__main__' has no attribute '<lambda>'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/$HOME/lib/python/cpython/Lib/multiprocessing/queues.py", line 262, in _feed
    obj = _ForkingPickler.dumps(obj)
  File "/$HOME/lib/python/cpython/Lib/multiprocessing/reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
_pickle.PicklingError: Can't pickle <function <lambda> at 0x7f2119283350>: it's not found as __main__.<lambda>

>>>

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

[Viicos]

I'm wondering if this relates to #129614, where I noticed the same indentation of some traceback.

[sergey-miryanov]

It works fine on Windows 11 x64 with current main:

➜ .\python.bat 
Running Debug|x64 interpreter...
Python 3.14.0a6+ (heads/main-dirty:06822bfbf62, Apr  3 2025, 01:20:23) [MSC v.1943 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import multiprocessing
>>> q = multiprocessing.Queue()
>>> try:
...     q.put(lambda : None)
...     print("Success!")
... except Exception:
...     print("Failure")
... 
Success!
AttributeError: module '__main__' has no attribute '<lambda>'

During handling of the above exception, another exception occurred:

>>> Traceback (most recent call last):
  File "D:\Sources\_pythonish\cpython\Lib\multiprocessing\queues.py", line 262, in _feed
    obj = _ForkingPickler.dumps(obj)
  File "D:\Sources\_pythonish\cpython\Lib\multiprocessing\reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
    _pickle.PicklingError: Can't pickle <function <lambda> at 0x00000200859BDFD0>: it's not found as __main__.<lambda>

[sergey-miryanov]

Hm, not fine:

➜ .\python.bat
Running Debug|x64 interpreter...
Python 3.14.0a6+ (heads/main:06822bfbf62, Apr  3 2025, 01:25:18) [MSC v.1943 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import multiprocessing
>>> q = multiprocessing.Queue()                                                                                                                                                                                                                                                                                                                                                                
>>> try:                                                                                                                                                                                                                                                                                                                                                                                       
...     q.put(lambda : None)
...     print("Success!")
... except Exception:
...     print("Failure")
... 
Success!
AttributeError: module '__main__' has no attribute '<lambda>'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:\Sources\_pythonish\cpython\Lib\multiprocessing\queues.py", line 262, in _feed
    obj = _ForkingPickler.dumps(obj)
>>>   File "D:\Sources\_pythonish\cpython\Lib\multiprocessing\reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
_pickle.PicklingError: Can't pickle <function <lambda> at 0x000001E63FAE9E50>: it's not found as __main__.<lambda>

[picnixz]

It might be because of the start method. Linux start method is forkserver while windows is spawn. I'll check that in a few hours. EDIT: didn't matter.

[picnixz]

@sergey-miryanov

Hm, not fine:

I'm not sure but what do you mean by "not fine"? it looks fine on Windows. It could entirely be a Linux-specific issue btw.

[sergey-miryanov]

@picnixz

[picnixz]

Oh thank you for pointing out this. I totally missed it!