class Delegator:

    # The cache is only used to be able to change delegates!

    def __init__(self, delegate=None):
        self.delegate = delegate
        self.__cache = set()

    def __getattr__(self, name):
        attr = getattr(self.delegate, name) # May raise AttributeError
        setattr(self, name, attr)
        self.__cache.add(name)
        return attr

    def __nonzero__(self):
        # this is needed for PyPy: else, if self.delegate is None, the
        # __getattr__ above picks NoneType.__nonzero__, which returns
        # False. Thus, bool(Delegator()) is False as well, but it's not what
        # we want.  On CPython, bool(Delegator()) is True because NoneType
        # does not have __nonzero__
        return True

    def resetcache(self):
        for key in self.__cache:
            try:
                delattr(self, key)
            except AttributeError:
                pass
        self.__cache.clear()

    def setdelegate(self, delegate):
        self.resetcache()
        self.delegate = delegate