To be used as a dictionary key, an object should be hashable. Hashable objects support the __hash__ method that returns an integer value. To get a hash of the value, the hash built-in function is used.
Built-in types that are not mutable are hashable by default. All custom objects are also hashable, but there is a catch. If you define __eq__ method for your custom type, then you should define such __hash__ that hash(a) == hash(b) for every a and b that are equal. Violating this rule may result in dictionary malfunctioning:
class A:
def __init__(self, x):
self.x = x
def __hash__(self):
return random.randrange(10000)
def __eq__(self, other):
return self.x == other.xIn : d = {}
In : d[A(2)] = 2
In : d.get(A(2), 0)
Out: 0
Mind that though once you define __eq__ in the class, the default __hash__ method is removed since the default implementation is no longer suitable (with it all values are unequal).
