python - 运行时错误 :freeze_support() on Mac

Question

我是python新手。我想学习如何在 python 中进行并行处理。我看到了下面的例子:

import multiprocessing as mp

np.random.RandomState(100)
arr = np.random.randint(0, 10, size=[20, 5])
data = arr.tolist()

def howmany_within_range_rowonly(row, minimum=4, maximum=8):
    count = 0
    for n in row:
    if minimum <= n <= maximum:
        count = count + 1
    return count

pool = mp.Pool(mp.cpu_count())

results = pool.map(howmany_within_range_rowonly, [row for row in data])

pool.close()

print(results[:10])

但是当我运行它时，发生了这个错误:

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.

我该怎么办？

Answer 1

如果您将所有内容都放在这个 if __name__ == "__main__" 内的全局范围内如下 block ，您应该会发现您的程序的行为与您预期的一样:

def howmany_within_range_rowonly(row, minimum=4, maximum=8):
    count = 0
    for n in row:
    if minimum <= n <= maximum:
        count = count + 1
    return count

if __name__ == "__main__":   
    np.random.RandomState(100)
    arr = np.random.randint(0, 10, size=[20, 5])
    data = arr.tolist()  
    pool = mp.Pool(mp.cpu_count())

    results = pool.map(howmany_within_range_rowonly, [row for row in data])

    pool.close()

    print(results[:10])

如果没有这种保护，如果您当前的模块是从不同的模块导入的，您的多处理代码将被执行。这可能发生在在另一个池中生成的非主进程中，并且不允许从子进程中生成进程，因此我们防止了这个问题。