settest.py

class C:
    def __init__(self, x):
        self.x = x
    def __eq__(self, y):  # crucial!
        return self.x == y.x
    def __hash__(self):
        return hash(self.x)

x = set()
a = C(2)
b = C(2)
x.add(a)
x.add(b)
print(x)