Class Inheritance Basics

Class Inheritance is a way to extend a class. The new class (child class) builds or overwrites logic from the parent class (super class).

The syntax for class extension is:

class ChildClass (ParentClass)

Overwriting methods:

Overwritten method is a method that has been inherited from the parent but it's logic is changed in the child.

Syntax for overwriting function:

class ParentClass: def funcName(args): # code class ChildClass (ParentClass): def funcName(args): # overwriting function

Let's have a base class Vehicle and child classes - Car, Scooter, Bicycle. We'll set a simplified formula for the insurance cost.

class Vehicle: def getInsuranceCost(self, months): return 0 # Scooter inherits Vehicle. class Scooter (Vehicle): # overwriting method def getInsuranceCost(self, months): return months * 15 # Car inherits Vehicle. class Car (Vehicle): # overwriting method def getInsuranceCost(self, months): return months * 25 # Bicycle inherits Vehicle. class Bicycle (Vehicle): pass # pass is used to avoid exception when class is empty # the class uses getInsuranceCost() # inherited from Vehicle. scooter = Scooter() print("Scooter Year Insurance:", scooter.getInsuranceCost(12), "$") car = Car() print("Car Year Insurance:", car.getInsuranceCost(12), "$") bike = Bicycle() print("Bicycle Year Insurance:", bike.getInsuranceCost(12), "$")

Output:

Scooter Year Insurance: 180 $ Car Year Insurance: 300 $ Bicycle Year Insurance: 0 $

Explanation:

• At line 6 the class Scooter inherits the Vehicle class. And at lines 8 and 9 the function getInsuranceCost is overwritten.
• At line 13 the class Car extends the Vehicle class. The Car class overwrites getInsuranceCost function at lines 15 - 16.
• At line 19 the Bicycle class extends Vehicle. It doesn't overwrite the getInsuranceCost method. When called (line 32) the code from the base class is executed.

Extending functions and constructors:

A child's class constructor can call parent's class constructor via the super keyword.

Let's have a base class Vehicle and child class Car. The child class will extend it's parent constructor and a function called representation.

class Vehicle: def __init__(self, brandValue, modelValue): self.brand = brandValue self.model = modelValue print("Vehicle constructor") def representation(self): print("Brand:", self.brand) class Car (Vehicle): def __init__(self, brandValue, modelValue, horsePowersValue): super().__init__(brandValue, modelValue) print("Car constructor") self.horsePowers = horsePowersValue def representation(self): super().representation() print("Model:", self.model) print("Horse Powers:", self.horsePowers) sampleBicycle = Vehicle("Merida ", "ONE-FORTY 800") sampleBicycle.representation() print("--------------------") fordCar = Car("Ford", "Fiesta", 80) fordCar.representation() print("--------------------") toyotaCar = Car("Toyota", "Corolla", 100) toyotaCar.representation()

Output:

Vehicle constructor Brand: Merida -------------------- Vehicle constructor Car constructor Brand: Ford Model: Fiesta Horse Powers: 80 -------------------- Vehicle constructor Car constructor Brand: Toyota Model: Corolla Horse Powers: 100

Explanation:

• At line 11 the class Car inherits Vehicle.
• The Car class constructor calls the Vehicle constructor at line 13 via the super keyword. When car object is created (lines 28 and 32) the code from the parent's class constructor (lines 3 - 6) executes first and afterwards the code from child's constructor (lines 13 - 16).
• At line 19 the representation function of the parent class is called. When representation function is called for a Vehicle object at line 25, only the brand is printed.
• When representation function is called for a Car object at lines 29 and 33, all 3 properties (make, model, horsePowers) are printed.