Fix Python – What’s a correct and good way to implement __hash__()?

Question

Asked By – user229898

What’s a correct and good way to implement __hash__()?

I am talking about the function that returns a hashcode that is then used to insert objects into hashtables aka dictionaries.

As __hash__() returns an integer and is used for “binning” objects into hashtables I assume that the values of the returned integer should be uniformly distributed for common data (to minimize collisions).
What’s a good practice to get such values? Are collisions a problem?
In my case I have a small class which acts as a container class holding some ints, some floats and a string.

Now we will see solution for issue: What’s a correct and good way to implement __hash__()?


Answer

An easy, correct way to implement __hash__() is to use a key tuple. It won’t be as fast as a specialized hash, but if you need that then you should probably implement the type in C.

Here’s an example of using a key for hash and equality:

class A:
    def __key(self):
        return (self.attr_a, self.attr_b, self.attr_c)

    def __hash__(self):
        return hash(self.__key())

    def __eq__(self, other):
        if isinstance(other, A):
            return self.__key() == other.__key()
        return NotImplemented

Also, the documentation of __hash__ has more information, that may be valuable in some particular circumstances.

This question is answered By – John Millikin

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0