with_classmethod.py

class Car(Model):
    make = CharField()
    model = CharField()
    year = IntegerField()

    @classmethod
    def add_car(cls, make, model, year):
        try:
            cls.create(make=make, model=model, year=year)
        except IntegrityError:
            raise ValueError("That car already exists")

without_classmethod.py

class Car(Model):
    make = CharField()
    model = CharField()
    year = IntegerField()



def add_car(make, model, year):
    try:
        Car.create(make=make, model=model, year=year)
    except IntegrityError:
        raise ValueError("That car already exists")