types.py 文件源码

def __init__(cls, name, bases, dictionary):
        if type(cls.__init__) is UnboundMethodType:
            initializer = cls.__init__
        elif type(cls.__new__) is FunctionType:
            initializer = cls.__new__
        else:
            # noinspection PyUnusedLocal
            def initializer(self, *args, **kw): pass

        # noinspection PyShadowingNames
        @preserve_signature(initializer)
        def instance_creator(cls, *args, **kw):
            key = (args, tuple(sorted(kw.iteritems())))
            try:
                hash(key)
            except TypeError:
                raise TypeError('cannot have singletons for classes with unhashable arguments')
            if key not in cls.__instances__:
                cls.__instances__[key] = super(Singleton, cls).__call__(*args, **kw)
            return cls.__instances__[key]

        super(Singleton, cls).__init__(name, bases, dictionary)
        cls.__instances__ = {}
        cls.__instantiate__ = instance_creator.__get__(cls, type(cls))  # bind instance_creator to cls