eq.py

class Kasper:
    def __init__(self, v):
        self.v = v

    def __eq__(self, other):
        return self.v == other.v

d = {}
d[Kasper(4)] = 5 # TypeError: unhashable type: 'Kasper'

print(Kasper(4) in d)

hash+eq.py

class Kasper:
    def __init__(self, v):
        self.v = v

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

    def __eq__(self, other):
        return self.v == other.v

d = {}
d[Kasper(4)] = 5

print(Kasper(4) in d) # True, works fine

hash.py

class Kasper:
    def __init__(self, v):
        self.v = v

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

d = {}
d[Kasper(4)] = 5

print(Kasper(4) in d) # False, not ok