Python multiprocessing only works in "fork"

System details:

Positron and OS details:

Positron Version: 2025.04.0 (Universal) build 64
Code - OSS Version: 1.96.0
Commit: b25e18713d464fb01352d31f7a165f1fb79d42c4
Date: 2025-03-10T02:32:22.057Z (5 days ago)
Electron: 32.2.6
Chromium: 128.0.6613.186
Node.js: 20.18.1
V8: 12.8.374.38-electron.0
OS: Darwin arm64 24.1.0

Interpreter details:

Python 3.13.2 | packaged by conda-forge | (main, Feb 17 2025, 14:02:48) [Clang 18.1.8 ]

Describe the issue:

When trying to run python code using the multiprocessing library, I get errors when it uses the default "spawn" start mode, and only runs properly in "fork"

Steps to reproduce the issue:

Example 1:

import multiprocessing as mp

def foo(q):
    q.put('hello')

if __name__ == '__main__':
    mp.set_start_method('spawn')
    q = mp.Queue()
    p = mp.Process(target=foo, args=(q,))
    p.start()
    print(q.get())
    p.join()

Example 2:

import multiprocessing as mp

def foo(q):
    q.put('hello')

if __name__ == '__main__':
    mp.set_start_method('fork')
    q = mp.Queue()
    p = mp.Process(target=foo, args=(q,))
    p.start()
    print(q.get())
    p.join()

Expected or desired behavior:

If I am understanding the documentation correctly, the default is "spawn" as thats safer. Would have to update all the code to use "fork" to operate properly in positron. Works fine in Rstudio.

https://docs.python.org/3/library/multiprocessing.html#module-multiprocessing

Were there any error messages in the UI, Output panel, or Developer Tools console?

Traceback (most recent call last):
  File "<string>", line 1, in <module>
    from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=87, pipe_handle=91)
                                                  ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nate/anaconda3/envs/py313/lib/python3.13/multiprocessing/spawn.py", line 122, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/Users/nate/anaconda3/envs/py313/lib/python3.13/multiprocessing/spawn.py", line 132, in _main
    self = reduction.pickle.load(from_parent)
AttributeError: Can't get attribute 'foo' on <module '__main__' (<class '_frozen_importlib.BuiltinImporter'>)>

Start by reproducing the two multiprocessing examples from the issue in Positron with Python 3.13, then inspect the reported spawn traceback and how Python code is launched. Done means the spawn example runs successfully and prints "hello" without requiring the fork start method.