Python 3.4+multiprocessingfunction

multiprocessing.set_start_method()

start methods

Sets the global start method for child processes: 'spawn', 'fork' or 'forkserver'. Call it at most once, under if __name__ == '__main__'.

Syntax

multiprocessing.set_start_method(method, force=False)

A second call raises RuntimeError "context has already been set", even if it was not you but get_start_method() or creating a Queue that fixed the method. force=True overrides it, but that signals a global setting is the wrong tool: in a library or importable code use get_context('spawn'). Called outside the __main__ guard under spawn, it also runs in every child, raising RuntimeError again.

Examples

import multiprocessing as mp
if __name__ == '__main__':
    mp.set_start_method('spawn')
    print(mp.get_start_method())  # → spawn
    try:
        mp.set_start_method('spawn')
    except RuntimeError as e:
        print(e)  # → context has already been set
    mp.set_start_method('spawn', force=True)
    print('forced')  # → forced

See also

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