Inheritance

Inheritance

When we create classes we can inherit methods and attributes from other already existing classes. This allows us to create a variety of different sub-classes or child classes based off of what is known as a parent class or super class.

The following words are all equivalent and mean a class that inherits from another:

• child class
• subclass
• derived class
• concrete class

The following words are all equivalent and mean a class that is inherited from:

• parent class
• super class
• abstract class

When we inherit from a class all the methods and attributes of the parent class are passed down to the child class.

class Animal():
    def __init__(self, name, age):
        self.name = name
        self.age = age 

    def speak(self):
        print("I am", self.name, "and I am", self.age, "years old")


class Dog(Animal):
    def __init__(self, name, age)
        self.name = name
        self.age = age
        self.type = "dog"

    # Since we inherit from the animal class we can use the method speak on Dog objects


tim = Dog("Tim", 5)
tim.speak() # This will print "I am Tim and I am 5 years old" 

Calling a Super-Classes Constructor

Often times when we create sub-classes we want to initialize them in a similar same way as the parent class. To save ourselves from repeating code we can call the parents init function from inside of our childs class init like so:

class Animal():
    def __init__(self, name, age):
        self.name = name
        self.age = age 

    def speak(self):
        print("I am", self.name, "and I am", self.age, "years old")


class Dog(Animal):
    def __init__(self, name, age)
        super().__init__(name, age)  # This will call the Animal classes constructor method


tim = Dog("Tim", 5)
tim.speak() # This will print "I am Tim and I am 5 years old" 

Overriding Methods

Sometimes when we inherit from a parent class we want to have methods or attributes that have the same name as a method in the parent class but that have a different functionality. If we create a method or attribute inside of our child class with the same name as one in the parent it will override the parent class.

class Animal():
    def __init__(self, name, age):
        self.name = name
        self.age = age 

    def speak(self):
        print("I am", self.name, "and I am", self.age, "years old")


class Dog(Animal):
    def __init__(self, name, age)
        super.__init__(name, age)  # This will call the Animal classes constructor method
    
    def speak(self):
        print("I am a Dog")


tim = Dog("Tim", 5)
tim.speak() # This will print "I am a Dog"

Example

Below is a realistic example of inheritance and where you may use it.

class Veichle:
    def __init__(self, price, color):
        self.color = color
        self.price = price
        self.gas = 0

    def fillUpTank(self):
        self.gas = 100

    def emptyTank(self):
        self.gas = 0

    def gasLeft(self):
        return self.gas


class Truck(Veichle):
    def __init__(self, price, color, tires):
        super().__init__(price, color)
        self.tires = tires

    def beep(self):
        print("Honk honk")


class Car(Veichle):
    def __init__(self, price, color, speed):
        super().__init__(price, color)
        self.speed = speed

    def beep(self):
        print("Beep Beep")