How Can I Resolve Serialization Errors When Using Instance Methods with Python's Multiprocessing Pool.map()?

Serialization Issues in Multiprocessing with Instance Methods

When working with multiprocessing's Pool.map() function, users may encounter serialization errors when attempting to pickle instance methods. This issue arises when an object-oriented approach is employed, resulting in the error message:

PicklingError: Can't pickle : attribute lookup __builtin__.instancemethod failed

Understanding the Problem

To understand the problem, it's important to recognize that multiprocessing utilizes serialization (pickling) to transfer objects between processes. Bound methods, however, cannot be pickled because they are not plain functions. They rely on the object they are bound to, which is not serializable.

Resolving the Issue

To overcome this serialization issue, one needs to implement a workaround by registering a function with the copy_reg standard library method. This function will enable the pickling and unpickling of bound methods.

Example Solution

The following code provides an example of how to implement a solution using copy_reg:

import copy_reg
import types

def pickle_method(method):
    func_name = method.__func__.__name__
    cls = method.__self__.__class__
    return (_unpickle_method, (func_name, cls))

def _unpickle_method(func_name, cls):
    for cls in cls.__mro__:
        try:
            func = getattr(cls, func_name)
            break
        except AttributeError:
            pass

    return func

copy_reg.pickle(types.MethodType, pickle_method)

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