Running a singleton with multiprocessing in python throws error.

singleton_multiprocessing_error.py

import time
from joblib import Parallel, delayed

class OnlyOne:
    """Singleton Class, inspired by 
    https://python-3-patterns-idioms-test.readthedocs.io/en/latest/Singleton.html"""
    class __OnlyOne:
        def __init__(self, arg):
            if arg is None:
                raise ValueError("Pretend empty instantiation breaks code")
            self.val = arg
        def __str__(self):
            return repr(self) + self.val
    instance = None
    def __init__(self, arg=None):
        if not self.instance:
            self.instance = self.__OnlyOne(arg)
        else:
            self.instance.val = arg
    def __getattr__(self, name):
        return getattr(self.instance, name)


def worker(num):
    """Single worker function to run in parallel.
    
    Assume that this function has to do an empty 
    instantiation of the singleton.
    """
    one = OnlyOne()
    time.sleep(0.1)
    one.val += num
    return one.val


# Instantiate singleton
one = OnlyOne(0)
print(one.val)

# Try to run in parallel
# Will hit the ValueError that raises with 
# empty instantiation
res = Parallel(n_jobs=-1, verbose=10)(
    delayed(worker)(i) for i in range(10)
)
print(res)