Is it possible to decorate a Python class whose instances will be pickled?

Question

The following code run correctly:

import pickle

class MyClass():   
    def __init__(self, arg):
        self.arg = arg

a = MyClass('my arg')
with open('/home/mahikeulbody/mypickle', 'wb') as file:
    pickle.dump(a, file)

but adding a decorator to get a multiton class :

import pickle

def multiton(cls):
    instances = {}
    def getinstance(arg):
        if arg not in instances:
            instances[arg] = cls(arg)
        return instances[arg]
    return getinstance

@multiton
class MyClass():   
    def __init__(self, arg):
        self.arg = arg

a = MyClass('my arg')
with open('/home/michel/mypickle', 'wb') as file:
    pickle.dump(a, file)

produces the following error:

pickle.dump(a, file)
_pickle.PicklingError: Can't pickle <class '__main__.MyClass'>: it's not the same object as __main__.MyClass

What is wrong ?

Answer 1

Pickle must be able to load the class directly. Your decorator replaces the class with a factory function, making it impossible for pickle to import the class itself.

Use a separate factory function, not a decorator, returning a 'private' class (but still importable directly):

class _MyClass():   
    def __init__(self, arg):
        self.arg = arg

def MyClass(arg, instances={}):
    if arg not in instances:
        instances[arg] = _MyClass(arg)
    return instances[arg]