decorator_process_executor.py

import functools
from concurrent.futures import ProcessPoolExecutor, ThreadPoolExecutor


def check_only_integers(fn):
    """Very simple decorator that prevents other than integers to be passed
    to the decorated function. Just for demonstration purposes.
    """
    @functools.wraps(fn)
    def wrapped(*args):
        if not all([type(arg) == int for arg in args]):
            raise ValueError("Invalid param is not an integer")
        return fn(*args)

    return wrapped


class CheckOnlyIntegers:
    def __init__(self, fn):
        self.fn = fn

    def __call__(self, *args):
        if not all([type(arg) == int for arg in args]):
            raise ValueError("Invalid param is not an integer")
        return self.fn(*args)


# @check_only_integers # (this decorator does work)
@CheckOnlyIntegers
def add(x, y):
    return x + y

# Tests:
assert add(2, 3) == 5
assert add(9, 11) == 20


with ProcessPoolExecutor() as ex:  # Switch to ThreadPoolExecutor and it works!
    future = ex.submit(add, 2, 3)
    print(future.result())