import constraint as cst
p = cst.Problem()

class A():
    after = []
    before = ["B"]
class B():
    after = ["A"]
    before = ["C"]
class C():
    after = ["B"]
    before = []

l = [A(), B(), C()]

p.addVariables(l, range(len(l)))
for e in l:
    for other_e in l:
        for a in e.after:
            if a in [c.__name__ for c in other_e.__class__.__mro__]:
                p.addConstraint(lambda x,y: x > y, [e, other_e])
        for b in e.after:
            if b in [c.__name__ for c in other_e.__class__.__mro__]:
                p.addConstraint(lambda x,y: x < y, [e, other_e])
p.getSolution()