PicklingError: Cannot Pickle Lambda or Local Function

Why This Happens

Python's multiprocessing uses pickle to serialize objects between processes. Pickle cannot serialize lambdas, local functions, or closures. Only module-level functions work.

The Problem

from multiprocessing import Pool
def main():
    func = lambda x: x ** 2
    with Pool(4) as p:
        results = p.map(func, range(10))

The Fix

from multiprocessing import Pool

def square(x):
    return x ** 2

def main():
    with Pool(4) as p:
        results = p.map(square, range(10))

Step-by-Step Fix

1. 1

   Move functions to module level

   Define at top level, not inside another function.

2. 2

   Replace lambdas

   Convert to named functions at module level.

3. 3

   Use ThreadPoolExecutor

   Threads do not require pickling and work with lambdas.