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python - Multiprocessing Module Error: `if __name__ == '__main__':

问题描述

I'm trying to run a code in multiprocessing mode using all the threads available to my CPU (MacBook Pro 16 inch 2019) and am getting an error I don't get running the exact same code on a linux environment for my windows computer.

I have never seen this error before. I tried uninstalling and reinstalling the multiprocessing module, but that didn't help. What is causing this issue? Again, the exact same code works on a Linux environment.

    pool = Pool(processes = ncpus)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/pool.py", line 212, in __init__
    self._repopulate_pool()
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/pool.py", line 303, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/pool.py", line 326, in _repopulate_pool_static
    w.start()
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "/Users/aeglick/opt/anaconda3/lib/python3.8/multiprocessing/spawn.py", line 134, in _check_not_importing_main
    raise RuntimeError('''
RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

标签： pythonmultiprocessing

解决方案

You have to run your Pool inside the block __main__:

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':  # <- you have to execute your Pool from here
    with Pool(5) as p:
        print(p.map(f, [1, 2, 3]))

So you can't launch a multiprocess code from a Python/IPython console.

Read this: https://docs.python.org/3/library/multiprocessing.html#multiprocessing-programming

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