gray3.py

class Gray:
    def __init__(self, length):
        self.length = length

    def _gen(self, start, end, n):
        if n == 1:
            yield (start, )
            yield (end, )
        else:
            for s in Gray(n-1):
                yield (start,) + s
            for s in reversed(Gray(n-1)):
                yield (end,) + s

    def __iter__(self):
        return self._gen(0, 1, self.length)

    def __reversed__(self):
        return self._gen(1, 0, self.length)

if __name__ == '__main__':
    print(list(Gray(2)))
    print(list(Gray(3)))