CODEWITHSUNDEEP
pythonequalityhashable
Jonas Adler
Jonas Adler

Reputation: 10759

Recommended way to implement __eq__ and __hash__

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?

Upvotes: 24

Views: 20298

Answers (3)

tiantanshu
tiantanshu

Reputation: 13

I think you can use user defined hash function is better.

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, self.b) == (other.a, other.b)
    else:
        return NotImplemented

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

Upvotes: -3

Rebuttle
Rebuttle

Reputation: 1

Is that the equivalent of this one-liner eq?

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

Upvotes: -1

Jonas Adler
Jonas Adler

Reputation: 10759

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())

Upvotes: 34

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