class main_class:
    def __init__(self, fd):
        self.fd = fd

class sub_class(main_class):
    def __init__(self, fd = "cat"):
        self.fd = fd

obj1 = main_class("dog")
obj2 = sub_class("eel")
obj3 = sub_class()

print(obj1.fd)
print(obj2.fd)
print(obj3.fd)