How can I make a class decorator not break isinstance function?

Question

I'm creating a singleton decorator for tests, but when I ask if an object is of instance of the original class it returns false.

In the example I'm decorating a counter class to create a singleton, so every time if I get the value it returns the next number no matter what instance of the object calls it. the code pretty much works but the function isinstance seems to break, I tried using functools.update_wrapper but I don't know if I can get isinstance function to recognize Singleton as Counter (in the following code) as long as when I ask for Counter the code actually returns Singleton.

decorator

def singleton(Class):

    class Singleton:
        __instance = None

        def __new__(cls):
            if not Singleton.__instance:
                Singleton.__instance = Class()

            return Singleton.__instance

    #update_wrapper(Singleton, Class, 
    #    assigned=('__module__', '__name__', '__qualname__', '__doc__', '__annotation__'), 
    #    updated=()) #doesn't seems to work
    return Singleton

decorated class

@singleton
class Counter:
    def __init__(self):
        self.__value = -1
        self.__limit = 6

    @property
    def value(self):
        self.__value = (self.__value + 1) % self.limit
        return self.__value

    @property
    def limit(self):
        return self.__limit

    @limit.setter
    def limit(self, value):
        if not isinstance(value, int):
            raise ValueError('value must be an int.')

        self.__limit = value

    def reset(self):
        self.__value = -1

    def __iter__(self):
        for _ in range(self.limit):
            yield self.value

    def __enter__(self):
        return self

    def __exit__(self,a,b,c):
        pass

test

counter = Counter()
counter.limit = 7
counter.reset()
[counter.value for _ in range(2)]

with Counter() as cnt:
    print([cnt.value for _ in range(10)]) #1

print([counter.value for _ in range(5)]) #2
print([val for val in Counter()]) #3

print(Counter) #4
print(type(counter)) #5 
print(isinstance(counter, Counter)) #6

output:

#1 - [2, 3, 4, 5, 6, 0, 1, 2, 3, 4]
#2 - [5, 6, 0, 1, 2]
#3 - [3, 4, 5, 6, 0, 1, 2]
#4 - <class '__main__.singleton.<locals>.Singleton'>
#5 - <class '__main__.Counter'>
#6 - False

(with the update wrapper uncommented)

#1 - [2, 3, 4, 5, 6, 0, 1, 2, 3, 4]
#2 - [5, 6, 0, 1, 2]
#3 - [3, 4, 5, 6, 0, 1, 2]
#4 - <class '__main__.Counter'>
#5 - <class '__main__.Counter'>
#6 - False

Answer 1

You can use the singleton class decorator in the Python Decorator Library .

It works because it modifies the existing class (replaces the __new__() method) instead of replacing it with a completely separate class as is being done in the code in your question.

import functools

# from https://wiki.python.org/moin/PythonDecoratorLibrary#Singleton
def singleton(cls):
    ''' Use class as singleton. '''

    cls.__new_original__ = cls.__new__

    @functools.wraps(cls.__new__)
    def singleton_new(cls, *args, **kw):
        it =  cls.__dict__.get('__it__')
        if it is not None:
            return it

        cls.__it__ = it = cls.__new_original__(cls, *args, **kw)
        it.__init_original__(*args, **kw)
        return it

    cls.__new__ = singleton_new
    cls.__init_original__ = cls.__init__
    cls.__init__ = object.__init__

    return cls

With it, I get the following output (note the last line):

[2, 3, 4, 5, 6, 0, 1, 2, 3, 4]
[5, 6, 0, 1, 2]
[3, 4, 5, 6, 0, 1, 2]
<class '__main__.Counter'>
<class '__main__.Counter'>
True

Answer 2

Not better than the above, but slightly simpler and easier to remember if you ever need to do this from memory at a later point:

def singleton(Class, *initargs, **initkwargs):
    __instance = Class(*initargs, **initkwargs)
    Class.__new__ = lambda *args, **kwargs: __instance
    Class.__init__ = lambda *args, **kwargs: None
    return Class