How to implement a good __hash__ function in python

Question

When implementing a class with multiple properties (like in the toy example below), what is the best way to handle hashing?

I guess that the __eq__ and __hash__ should be consistent, but how to implement a proper hash function that is capable of handling all the properties?

class AClass:
  def __init__(self):
      self.a = None
      self.b = None

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

  def __ne__(self, other):
    return not self.__eq__(other)

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

I read on this question that tuples are hashable, so I was wondering if something like the example above was sensible. Is it?

Answer 1

Documentation for object.hash(self)

The only required property is that objects which compare equal have the same hash value; it is advised to somehow mix together (e.g. using exclusive or) the hash values for the components of the object that also play a part in comparison of objects.

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

Answer 2

__hash__ should return the same value for objects that are equal. It also shouldn't change over the lifetime of the object; generally you only implement it for immutable objects.

A trivial implementation would be to just return 0. This is always correct, but performs badly.

Your solution, returning the hash of a tuple of properties, is good. But note that you don't need to list all properties that you compare in __eq__ in the tuple. If some property usually has the same value for inequal objects, just leave it out. Don't make the hash computation any more expensive than it needs to be.

Edit: I would recommend against using xor to mix hashes in general. When two different properties have the same value, they will have the same hash, and with xor these will cancel eachother out. Tuples use a more complex calculation to mix hashes, see tuplehash in tupleobject.c.