我就是这么想的:

class Pair():
    def __init__(self, key, value):
        self.key = key
        self.value = value


class dictionary():
    def __init__(self):
        self.items = []

    def add(self, objects):
        self.items.append(objects)

    def __keys__(self):
        return iterator(self, 'keys')

    def __values__(self):
        return iterator(self, 'values')

class iterator():
    def __init__(self, to_be_iterated , over_what):
        self.to_be_iterated = to_be_iterated
        self.over_what = over_what


    def __iter__(self):
        self.index = -1
        return self

    def __next__(self):
        self.index += 1
        if self.over_what == 'keys':
            try:
                    return self.to_be_iterated.items[self.index].key
            except Exception:
                raise StopIteration

        elif self.over_what == 'values':
            try:
                    return self.to_be_iterated.items[self.index].value
            except Exception:
                raise StopIteration


collins = dictionary()

collins.add(Pair('up', 'above'))
collins.add(Pair('down', 'below'))

for i in collins.__keys__():
    print(i)

for i in collins.__values__():
    print(i)