Why Do I Get a RuntimeError When Using Multiprocessing on Windows?

Windows RuntimeError Using Multiprocessing

This error arises when attempting to launch Python multiprocessing processes on Windows without using the proper initialization idiom.

In the question, a Python program attempts to launch threads and processes from a separate module but encounters a RuntimeError. The cause is that Windows subprocesses implicitly import the main module upon initialization. Without the proper guard, it leads to recursive subprocess creation.

Solution:

To resolve the error, it is necessary to add an if name == '__main__' guard in the main module to prevent multiple process instances.

Modified Main Module (testMain.py):

if __name__ == '__main__':    
    import parallelTestModule

    extractor = parallelTestModule.ParallelExtractor()
    extractor.runInParallel(numProcesses=2, numThreads=4)

In this modified version, the main module explicitly checks if it is the main process before executing the parallel code. This ensures that the subprocesses are not recursively created and avoids the RuntimeErorr.

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