Why Can't I Pickle My Function in Multiprocessing?

Multiprocessing Error: "Can't Pickle ": Unveiling the Issue

Your code encounters a "PicklingError: Can't pickle ": attribute lookup __builtin__.function failed." When trying to use multiprocessing to apply a function asynchronously. This error arises because the function being pickled is not defined at the top level of the module.

Understanding the Pickling Hierarchy

Python's pickling mechanism enables the serialization of objects into a byte stream for later deserialization. However, not all objects can be pickled. Notably, functions are only picklable if they are defined at the top level of a module. This is because the pickle function requires access to the function's globals, which are not available when defined within another function or class.

Your Code's Situation

While your function is defined at the top level, it calls other functions that may not be top-level definitions. This can create a recursive dependency, making the pickling of the top-level function impossible.

A Solution: Move the Function Definition

The key to resolving the issue is to ensure that the function you're attempting to pickle is defined at the top level of the module. If necessary, move the function's definition outside of any classes or other functions.

Here's an example:

# Original code
class Foo:
    @staticmethod
    def work(self):
        pass

# Updated code
def work(foo):
    foo.work()

Wrap-up

By understanding the pickling process and its limitations, you can avoid errors related to the pickling of functions. Remember to define functions at the top level of the module to ensure their successful pickling when using multiprocessing.

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