AnymalExample.py

class Animal:
    def __init__(self):
        self.firstname = 'maci'
        self.lastname = 'laci'
        self.fullname = self.firstname + self.lastname

    def __setattr__(self, __name: str, __value: any) -> None:
        super().__setattr__(__name, __value)
        try:
            if __name == 'firstname' or __name == 'lastname':
                self.fullname = self.firstname + self.lastname
        except:
            pass

class Dog(Animal):
    def __init__(self):
        super().__init__()
        self.firstname = 'Bello'
        self.lastname = 'Kutyus'


oneDog = Dog()
print (oneDog.fullname)