class Test(object):
    _instance = None

    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super(Test, cls).__new__(*args, **kwargs)
        return cls._instance


    def __init__(self):
        super(Test, self).__init__()

    @classmethod
    def foo(cls):
        print("classmethod")

    def foo(self):
        print("method", self)


Test.foo()
Test().foo()