Why Does Python Multiprocessing Cause a RuntimeError on Windows When Threads Are Managed in a Separate Module?

Fixing RuntimeError on Windows While Using Python Multiprocessing

Python multiprocessing can encounter a "RuntimeError" on Windows systems if the main module is not properly configured. The error message typically suggests using the "freeze_support()" function to rectify this issue. However, let's delve into a specific scenario where the threads are managed in a separate module, leading to the same error.

The code involves a main module ("testMain.py") that imports the module handling processes and threads ("parallelTestModule.py") and invokes its "runInParallel" method. In "parallelTestModule.py," the processes are defined within the "ParallelExtractor" class, which initializes processes using the "Process" class from the "multiprocessing" module.

The crux of the problem lies in the lack of an "if name == '__main__':" guard in the main module ("testMain.py"). This guard ensures that the subprocesses do not import the main module recursively. When executed on Windows, subprocesses execute the main module at the start, causing a recursive loop.

To resolve the issue, you need to insert this guard in the main module ("testMain.py"):

import parallelTestModule

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

With this modification, the main module will act as an entry point only when the program is run directly, preventing recursive execution of subprocesses and allowing multiprocessing to function correctly on Windows.

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