class Base:

    def __init__(self, a, b, c=None):
        self.a = a
        self.b = b
        self.c = c

    def __str__(self):
        return '{}, {}, {}'.format(self.a, self.b, self.c)


class Derived(Base):

    def __init__(self, a, b, c, d):
         super().__init__(a, b, c)    # pass the to the Base initializer
         self.d = d                   # add the extra variable

    def __str__(self):
        # Here string representation of the Base part with added self.d
        return super().__str__() + ', {}'.format(self.d)


if __name__ == '__main__':

    bas = Base(1, 2, 3)
    print(bas)

    der = Derived(1, 2, 3, 4)
    print(der)