Context manager for temporarily overriding Python forking behavior.

temporary_fork_behavior_setter.py

from contextlib import contextmanager
from multiprocessing import get_start_method, set_start_method

from typing import Literal


@contextmanager
def ForkingBehavior(
    *,
    start_method: Literal['spawn', 'fork', 'forkserver'],
    force: bool,
):
    prev_start_method = get_start_method()
    set_start_method(start_method, force=force)
    try:
        yield
    finally:
        set_start_method(prev_start_method, force=True)


if __name__ == "__main__":
    print(f"before:  {get_start_method()=}")
    try:
        with ForkingBehavior(start_method='forkserver', force=True):
            print(f"inside:  {get_start_method()=}")
            raise ValueError
    finally:
        print(f"after:   {get_start_method()=}")
# 
# Produces:
#
#     before:  get_start_method()='spawn'
#     inside:  get_start_method()='forkserver'
#     after:   get_start_method()='spawn'
#     Traceback (most recent call last):
#       File "temporary_fork_behavior_setter.py.py", line 26, in <module>
#         raise ValueError
#     ValueError