Example of overriding object equality in python

equality_override.py

from collections import Counter


class A(object):
    def __init__(self, name, val):
        self.name = name
        self.val = val

    def __repr__(self):
        return 'A(val={})'.format(self.val)

    def __eq__(self, other):
        return self.val == other.val

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


def main():
    x = A('one', 1)
    y = A('uno', 1)
    z = A('two', 2)
    w = A('dos', 2)

    print(x == y)  # True
    print(x == z)  # False
    
    # only one of x,y and z,w will remain in sets so name
    # information will be lost in both containers
    print(set([x, y, z, w]))  # set([A(val=1), A(val=2)])
    print(Counter([x, y, z, w]))  # Counter({A(val=1): 2, A(val=2): 2})

    x.name = '1'  # This is acceptable

    x.val = 3  # This *must not* happen!
    # Behavior of previous objects is undefined after this update

if __name__ == '__main__':
    main()