Why Do I Get 'RuntimeError: Attempt to start a new process...' When Using Multiprocessing in Python on Windows?

Runtime Errors in Python Multiprocessing on Windows

When executing multiprocessing code on Windows operating systems, you may encounter the following error message:

RuntimeError:
            Attempt to start a new process before the current process
            has finished its bootstrapping phase.
            This probably means that you are on Windows 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 a Windows executable.

Cause of the Error

In Windows, subprocesses created through the multiprocessing module will inherit the environment of the main process. When a subprocess is launched, it will attempt to import the main module, which can lead to recursive execution of the code. To prevent this, you need to add an if __name__ == '__main__' guard to the main module.

Solution

To resolve the issue, modify the main module as follows:

import parallelTestModule

if __name__ == '__main__':    
    extractor = parallelTestModule.ParallelExtractor()
    extractor.runInParallel(numProcesses=2, numThreads=4)

Explanation

The if __name__ == '__main__' condition checks if the code is being executed from the main module directly. If this condition is met, it means that the script is not being imported by another module. In this case, you can safely create subprocesses within the main module, as it ensures that the code will not be executed recursively.

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