How to convert an custom class object to a tuple in Python?

Question

If we define __str__ method in a class:

class Point():
    def __init__(self, x, y):
        self.x = x
        self.y = y


    def __str__(self, key):
        return '{},{}'.format(self.x, self.y)

So we can convert its object to str immediately:

a = Point(1, 1)
b = str(a)
print(b)

But as far as I know, there is not such __tuple__ magic method, so I do not know how to define a class which can pass to tuple() so that we can convert its object to tuple immediately.