Singleton metaclass

singleton.py

class Singleton(type):
    """A singleton metaclass.

    Usage:
    class Foo(object):
        __metaclass__ = Singleton

    assert Foo() is Foo()
    """

    def __init__(self, *args, **kwargs):
        self.__instance = None
        super(Singleton, self).__init__(*args, **kwargs)

    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super(Singleton, self).__call__(*args, **kwargs)

        return self.__instance