`multiprocessing.Pool` lambda initializer raises PicklingError in 3.14

[bgilbert]

Bug report

Bug description:

import multiprocessing
multiprocessing.Pool(None, lambda: None)

This works in Python versions up to 3.13.9, but fails in 3.14.0:

AttributeError: module '__main__' has no attribute '<lambda>'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/bgilbert/tmp/t/pool.py", line 2, in <module>
    multiprocessing.Pool(None, lambda: None)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
                context=self.get_context())
  File "/usr/lib64/python3.14/multiprocessing/pool.py", line 215, in __init__
    self._repopulate_pool()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.14/multiprocessing/pool.py", line 306, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
                                        self._processes,
                                        ^^^^^^^^^^^^^^^^
    ...<3 lines>...
                                        self._maxtasksperchild,
                                        ^^^^^^^^^^^^^^^^^^^^^^^
                                        self._wrap_exception)
                                        ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/pool.py", line 329, in _repopulate_pool_static
    w.start()
    ~~~~~~~^^
  File "/usr/lib64/python3.14/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/context.py", line 300, in _Popen
    return Popen(process_obj)
  File "/usr/lib64/python3.14/multiprocessing/popen_forkserver.py", line 35, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/popen_forkserver.py", line 47, in _launch
    reduction.dump(process_obj, buf)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^
_pickle.PicklingError: Can't pickle <function <lambda> at 0x7ff55732b480>: it's not found as __main__.<lambda>
when serializing tuple item 2
when serializing dict item '_args'
when serializing multiprocessing.context.ForkServerProcess state
when serializing multiprocessing.context.ForkServerProcess object

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[duaneg]

This is because the default multiprocessing start method changed on Linux in that version. It is now forkserver, which requires pickling the state, where previously it was fork that does not. Calling multiprocessing.set_start_method('fork') will fix it:

import multiprocessing as mp
mp.set_start_method('fork')
with mp.Pool(None, lambda: None): pass