Implementation of the Singleton pattern in Python

Asked Dec 08 '15 at 11:57

Active Dec 08 '15 at 11:57

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As answered brilliantly by agf here, a possible implementation of a Singleton in Python could be using this metaclass:

class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

However, I do not understand the use of _instance. I know it's a dict used to store instances indexed by their class. However, it is declared as a class attribute (_instances = {}), and it is used as an attribute of cls (if cls not in cls._instances:). How can the two be the same?

edited May 23 '17 at 12:23

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asked Dec 08 '15 at 11:57

Mankalas

shouldn't `def __call__(...)` be decorated with `@classmethod`? – hiro protagonist Dec 08 '15 at 12:07
Well, if you decorate `__call__` with `@classmethod`, a `cls` attribute is a class attribute, as the parameter name would suggest. – DainDwarf Dec 08 '15 at 12:14

Implementation of the Singleton pattern in Python

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