Question

Suppose I have a namedtuple like this:

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

I want to implement a custom hash-function for this, so I create the following subclass:

class Edge(EdgeBase):
    def __hash__(self):
        return hash(self.left) * hash(self.right)

Since the object is immutable, I want the hash-value to be calculated only once, so I do this:

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

    def __hash__(self):
        return self._hash

This appears to be working, but I am really not sure about subclassing and initialization in Python, especially with tuples. Are there any pitfalls to this solution? Is there a recommended way how to do this? Is it fine? Thanks in advance.

Answer 1

edit for 2017: turns out namedtuple isn’t a great idea. attrs is the modern alternative.

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

    def __hash__(self):
        return self._hash

__new__ is what you want to call here because tuples are immutable. Immutable objects are created in __new__ and then returned to the user, instead of being populated with data in __init__.

cls has to be passed twice to the super call on __new__ because __new__ is, for historical/odd reasons implicitly a staticmethod.