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.
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.