Mastering Object-Oriented Python - Second Edition

Deciding what to hash

Not every object should provide a hash value. Specifically, if we're creating a class of stateful, mutable objects, the class should never return a hash value. There should not be an implementation of the __hash__() method.

Immutable objects, on the other hand, might sensibly return a hash value so that the object can be used as the key in a dictionary, or as a member of a set. In this case, the hash value needs to parallel the way the test for equality works. It's bad to have objects that claim to be equal and have different hash values. The reverse—objects with the same hash that are actually not equal – is acceptable and expected. Several distinct objects may happen to have overlapping hash values.

There are three ...