class NotAdder:
    def __init__(self):
        self.i = 0
    def __add__(self, other):
        return NotImplemented

class Adder:
    def __init__(self):
        self.i = 1
    def __add__(self, other: NotAdder):
        return self.i + other.i
    def __radd__(self, other: NotAdder):
        return self.i + other.i

if __name__ == '__main__':
    na = NotAdder()
    a = Adder()
    na + a

# if a + na => NotImplemented then
# na.__radd__(a)