class A:
    def __init__(self, a):
        self.a = a
    def some_method(self):
        print(self.a)

class B(A):
    def __init__(self, a, b):
        super().__init__(a)
        self.b = b

    def methodB(self):
        print("Method samo za B")
class C(A):
    def __init__(self, a, c):
        super().__init__(a)
        self.c = c


a = A(50)
b = B(30, 60)
c = C(60, 90)

lista = [a,b,c]

for element in lista :
    element.some_method()
    if isinstance(element, B):
        element.methodB()