Why Does Multiprocessing Example Result in \'AttributeError\' with Built-in Functions?

Why Multiprocessing Example Gives AttributeError

In an attempt to delve into multiprocessing, an individual encountered an AttributeError while adapting the introductory example from the documentation:

<code class="python">from multiprocessing import Pool
def f(x):
    return x*x

if __name__ == '__main__':
    with Pool(5) as p:
        print(p.map(f, [1, 2, 3]))</code>

The error: "AttributeError: can't get attribute 'f' on " perplexed the user.

To resolve this issue, it's important to understand that multiprocessing.Pool has a peculiar design feature. As noted in Python issue #25053, Pool sometimes falters when working with objects not defined in imported modules. As a workaround, you can define your function in a separate file and import the module.

Here's an example:

defs.py:

<code class="python">def f(x):
    return x*x</code>

run.py:

<code class="python">from multiprocessing import Pool
import defs

if __name__ == '__main__':
    with Pool(5) as p:
        print(p.map(defs.f, [1, 2, 3]))</code>

This modification should resolve the AttributeError. However, it's worth noting that the given example in the documentation might not be the most suitable for beginners due to this potential issue.

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