**If eq() method is not overriden, it returns `id(self) == id(other)`, which is the same as `self is other`, meaning all objects compare not equal by default.**

**Hashable object needs both hash() and eq() methods and it's hash value should never change. Objects that compare equal must have the same hash value, meaning default hash() that returns `id(self)` will not do. That is why Python automatically makes classes unhashable if you only implement eq().**