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I get a error when I run this code.

from multiprocessing import Process, cpu_count import time def counter(num): count = 0 while count < num: count += 1 def main(): print("cpu count:", cpu_count()) a = Process(target=counter, args=(500000000,)) b = Process(target=counter, args=(500000000,)) a.start() b.start() print("processing...") a.join() b.join() print("Done!") print("finished in:", time.perf_counter(), "seconds") main()

I was expecting Python to print up to 1000000000 but it just gives me a unexpected error. I am not sure what if name == "main": does so I have not used it.

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    "I am not sure what if __name__ == "__main__": does so I have not used it." Removing that would be the first problem :) See also Compulsory usage of if __name__=="__main__" in windows while using multiprocessing and linked questions. Commented Jul 22, 2023 at 0:34
  • removing what?? Commented Jul 22, 2023 at 0:37
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    but it just gives me a unexpected error Are we supposed to guess what the error is? Commented Jul 22, 2023 at 0:37
  • @SkrugNuggsta the if __name__ == "__main__": guard. Commented Jul 22, 2023 at 0:37
  • 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. Commented Jul 22, 2023 at 0:42

2 Answers 2

Reset to default
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Your problem is that you must write:

if __name__ == '__main__': main()

On non-Linux machines, Python starts new processes but having them re-read the main file. In your original code, each time a new process is started, it starts up another ten processes.

You need to make sure that main() is only executed once.

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"On non-Linux machines, Python starts new processes" fun fact, that fact this this isn't the default on unix systems is now considered a mistake. Current sites are on making spawn the default start method on unix systems as well in Python 3.14.
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If you want to measure a duration you need to note both start and end times then subtract one from the other.

Something like this:

from multiprocessing import Pool from time import perf_counter def counter(num): for _ in range(num): ... return num N = 500000000 NPROCS = 2 def main(): start = perf_counter() with Pool() as pool: for ar in [pool.apply_async(counter, (N,)) for _ in range(NPROCS)]: print(ar.get()) end = perf_counter() print(f'Duration {end-start:.2f}s') if __name__ == '__main__': main()

Output:

500000000 500000000 Duration 8.61s
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