Python __hash__ for equal value objects

Question

Say I have some Person entities and I want to know if one is in a list:

person in people?

I don't care what the 'object's ID' is, just that their properties are the same. So I put this in my base class:

# value comparison only
def __eq__(self, other):
    return (isinstance(other, self.__class__) and self.__dict__ == other.__dict__)

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

But to be able to test for equality in sets, I also need to define hash So...

# sets use __hash__ for equality comparison
def __hash__(self):
    return (
        self.PersonID,
        self.FirstName,
        self.LastName,
        self.etc_etc...
    ).__hash__()

The problem is I don't want to list every property, and I don't want to modify the hash function every time the properties change.

So is it okay to do this?

# sets use __hash__ for equality comparison
def __hash__(self):
    values = tuple(self.__dict__.values())
    return hash(values)

Is this sane, and not toooo much of a performance penalty? In a web-app situation.

Thanks muchly.

Answer 1

The unordered nature of dictionaries means that tuple(self.__dict__.values()) is prone to producing different results if the dicts happen to be ordered differently (which could happen, for example, if one had its attributes assigned in a different order).

Because your values are hashable, you could try this instead:

return hash(frozenset(self.__dict__.iteritems()))

Alternatively, note that __hash__ does not need to take everything into account because __eq__ will still be used to verify equality when the hash values compare equal. Therefore, you can probably get away with

return hash(self.PersonID)

assuming PersonID is relatively unique across instances.