How to provide additional initialization for a subclass of namedtuple?

Question

Suppose I have a namedtuple like this:

EdgeBase = namedtuple(\"EdgeBase\", \"left, right\")

I want to implement a custom hash-

Answer 1

The code in the question could benefit from a super call in the __init__ in case it ever gets subclassed in a multiple inheritance situation, but otherwise is correct.

class Edge(EdgeBase):
    def __init__(self, left, right):
        super(Edge, self).__init__(left, right)
        self._hash = hash(self.left) * hash(self.right)

    def __hash__(self):
        return self._hash

While tuples are readonly only the tuple parts of their subclasses are readonly, other properties may be written as usual which is what allows the assignment to _hash regardless of whether it's done in __init__ or __new__. You can make the subclass fully readonly by setting it's __slots__ to (), which has the added benefit of saving memory, but then you wouldn't be able to assign to _hash.