Inheritance in Python with Different Argument(s)

Question

When a subclass inherits from a superclass, must the subclass have all the arguments that the superclass has? E.g.: "Vehicle(colour, wheels, size)" is the superclass. Can I have a subclass that inherits everything from Vehicle except the "size" attribute/argument?

This is the parent class:

class LagStudent:

    def __init__(self, name, dept, cgpa, is_a_scholar):
        self.name = name
        self.dept = dept
        self.cgpa = cgpa
        self.is_a_scholar = is_a_scholar

Below are two subclasses:

class CovStudent(LagStudent):

    def __init__(self, name, dept, cgpa, is_a_scholar, honours):
        super().__init__(name, dept, cgpa, is_a_scholar)
        self.honours = honours


class OxStudent(CovStudent):

    def __init__(self, name, dept, cgpa, is_a_scholar):
        super().__init__(name, dept, cgpa, is_a_scholar) 

When I run the following...

student4 = OxStudent("Mark", "Mathematics", 4.62, True)
print(student4.is_a_scholar)

It gives Error:

TypeError: init() missing 1 required positional argument: 'honours'

Answer 1

We define the Vehicle superclass as follows, and Bicycle subclass as follows

class Vehicle:

    def __init__(self, colour, wheels, size):

        self.colour = colour
        self.wheels = wheels
        self.size = size

class Cycle(Vehicle):

    def __init__(self, colour, wheels, size):

        super().__init__(colour, wheels, 'small')

Here you can see that the Cycle subclass just takes in colors and wheels and passes it onto the superclass, and has a hardcoded size=small attribute. The subclass constructor calls the constructor of the superclass

To check what's going on, we can try as follows:

veh = Cycle('Black', 2)
print(veh.colour)
print(veh.wheels)
print(veh.size)
#Black
#2
#small

Update Based on OP's comment of not wanting to have a size attribute, you can assign a default value to the size attribute in the superclass, e.g. size=None and you won't need to pass it while instantiating the subclass.

class Vehicle:

    def __init__(self, colour, wheels, size=None):

        self.colour = colour
        self.wheels = wheels
        self.size = size

class Cycle(Vehicle):

    def __init__(self, colour, wheels):

        super().__init__(colour, wheels)

You can then call this as follows

veh = Cycle('Black', 2)
print(veh.colour)
print(veh.wheels)
print(veh.size)
#Black
#2
#None

You can see that the value of size is None, which is the default value we picked since we did not pass size explicitly, if we would have passed it, we would have got a value for the size attribute

Answer 2

A subclass certainly can have a different signature for the __init__ method from its superclass. It just needs to call the __init__ method of its superclass with what that method expects:

class Vehicle:
    def __init__(self, colour, wheels, size):
        self.colour = colour
        self.wheels = wheels
        self.size = size

class Car(Vehicle):
    def __init__(self, colour, wheels):
        super().__init__(colour, wheels, 'small')

print(Car('white', 4).size)

This outputs:

small