Recommended way to implement __eq__ and __hash__

问题:

The python documentation mentions that if you override __eq__ and the object is immutable, you should also override __hash__ in order for the class to be properly hashable.

In practice, when I do this I often end up with code like

class MyClass(object):
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __eq__(self, other):
        if type(other) is type(self):
            return (self.a == other.a) and (self.b == other.b)
        else:
            return False

    def __hash__(self):
        return hash((self.a, self.b))

This is somewhat repetitive, and there is a clear risk of forgetting to update one when the other is updated.

Is there a recommended way of implementing these methods together?

回答1:

Answering my own question. It seems one way of performing this is to define an auxillary __members function and to use that in defining __hash__ and __eq__. This way, there is no duplication:

class MyClass(object):
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __members(self):
        return (self.a, self.b)

    def __eq__(self, other):
        if type(other) is type(self):
            return self.__members() == other.__members()
        else:
            return False

    def __hash__(self):
        return hash(self.__members())