Python __hash__ for equal value objects
hash, python, set
Solution
The unordered nature of dictionaries means that `tuple(self.__dict__.values())` is prone to producing different results if the `dict`s 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__.items()))
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.
Problem
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.