How to implement a good __hash__ function in python [duplicate]

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__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.

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