cpython/Lib/idlelib/delegator.py

class Delegator:

    def __init__(self, delegate=None):
        self.delegate = delegate
        self.__cache = set()
        # Cache is used to only remove added attributes
        # when changing the delegate.

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

    def resetcache(self):
        "Removes added attributes while leaving original attributes."
        # Function is really about resetting delegator dict
        # to original state.  Cache is just a means
        for key in self.__cache:
            try:
                delattr(self, key)
            except AttributeError:
                pass
        self.__cache.clear()

    def setdelegate(self, delegate):
        "Reset attributes and change delegate."
        self.resetcache()
        self.delegate = delegate

if __name__ == '__main__':
    from unittest import main
    main('idlelib.idle_test.test_delegator', verbosity=2)