How to make an object properly hashable?

Question

Here is my code:

class Hero:     def __init__(self, name, age):         self.name = name         self.age = age      def __str__(self):         return self.name + str(self.age)      def __hash__(self):         print(hash(str(self)))         return hash(str(self))  heroes = set()  heroes.add(Hero('Zina Portnova', 16)) # gets hash -8926039986155829407 print(len(heroes)) # gets 1  heroes.add(Hero('Lara Miheenko', 17)) # gets hash -2822451113328084695 print(len(heroes)) # gets 2  heroes.add(Hero('Zina Portnova', 16)) # gets hash -8926039986155829407 print(len(heroes)) # gets 3! WHY? 

Why is this happening?
The 1st and the 3rd object have same content and same hash but len() tells about 3 unique objects?

Answer 1

You also need to define __eq__() in a compatible way with __hash__() – otherwise, equality will be based on object identity.

On Python 2, it is recommended you also define __ne__ to make != consistent with ==. On Python 3, the default __ne__ implementation will delegate to __eq__ for you.