02 - Intermediate Concepts

Section 02: Intermediate Concepts

• Inheritance
  • Overview
  • The "is a" Relationship
• Multi-Level Inheritance
  • Overview
  • Extends Class
• Multiple Inhertance (Diamond Inheritance)
  • Overview
  • Extends Class
• Method Resolution Order (MRO)

Inheritance

Overview

The diagram can give us a quick overview of what inheritance is.

• We have two classes A and B. Class A is the base class (also called parent class or super class) and class B is the derived class (also called child class or subclass).
• Inheritance is the concept that we can have a behavior which is common to multiple classes. Classes which inherit that behavior and also adds new functionality that is specific only to each particular class.

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The "is a" Relationship

A good example for demonstrating inheritance is a scenario including animals. Let's create the class to represent dogs and cats.

• The cats and dogs have much in common.
  • a list of traits: breed, color and weight...
  • a list of behavior: walking, eating and sleeping...
• The cats and dogs have some characteristics that make them different.
  • barks and meows

The relationship between a dog or a cat and an animal in general is known as an "is a" relationship.

class Animal:
    def __init__(self, breed, color, weight):
        self.breed = breed
        self.color = color
        self.weight = weight

    def walk(self):
        print("I am walking.")

    def eat(self):
        print("I am eating.")

    def sleep(self):
        print("I am sleeping.")


class Dog(Animal):
    def bark(self):
        print("I am barking.")


class Cat(Animal):
    def meow(self):
        print("I am meowing.")

Please note that in order to inherit from the Animal class, we have to define it before the child classes, otherwise it will not work.

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Multi-Level Inheritance

Overview

Multi-level Inheritance is used whenever you want to further extend the traits or behavior of a class. The diagram can give us a quick overview of what multi-level inheritance is.

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Extends Class

A good example of multi-level inheritance is a police dog that is a special dog with all the capabilities of a normal dog. But it also has the advantage of detecting drugs.

• A police dog is a special dog.
• Every dog is an animal.

class Animal:
    def __init__(self, breed, color, weight):
        self.breed = breed
        self.color = color
        self.weight = weight

    def walk(self):
        print("I am walking.")

    def eat(self):
        print("I am eating.")

    def sleep(self):
        print("I am sleeping.")


class Dog(Animal):
    def bark(self):
        print("I am barking.")


class PoliceDog(Dog):
    def __init__(self, breed, color, weight, hours_on_mission):
        super().__init__(breed, color, weight)
        self.hours_on_mission = hours_on_mission

    def detect_drugs(self):
        print("Sniff...sniff... I smell some weed here!")

Note that we need to call the constructor of the parent class by using the super() keyword.

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Multiple Inhertance (Diamond Inheritance)

Overview

Multiple Inheritance (or Diamond Inheritance) let a class inherit from more than one class simultaneously. The diagram can give us a quick overview of what multiple inheritance is.

It is maily used in the context of the so-called mixin classes. These mixin classes are not made to stand on their own and are only classes which store stateless behavior.

• Mixin Classes are not made to stand on their own.
• Mixin Classes are only classes which store stateless behavior.
• Mixin Classes don't have any "is-a" relationships.

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Extends Class

We have created the PoliceDog class in order to provide it with the ability of detecting drugs. If we want to do this by using a multiple inheritance and the mixing class, we have to create another class to store the ability to detect drugs.

class Animal:
    def __init__(self, breed, color, weight):
        self.breed = breed
        self.color = color
        self.weight = weight

    def walk(self):
        print("I am walking.")

    def eat(self):
        print("I am eating.")

    def sleep(self):
        print("I am sleeping.")


class Dog(Animal):
    def bark(self):
        print("I am barking.")


class DetectDrugsMixin:
    def detect_drugs(self):
        print("I'm detecting some drugs here!")


class PoliceDog(Dog, DetectDrugsMixin):
    def __init__(self, breed, color, weight, hours_on_mission):
        super().__init__(breed, color, weight)
        self.hours_on_mission = hours_on_mission

In order to inherit the detect_drugs behavior, we need to also inherit the DetectDrugsMixin class.

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Method Resolution Order (MRO)

The Method Resolution Order (MRO) is a very important concept when using multiple inheritance: "When inherited classes have the same methods or variables, which one is going to be used in the end?"

class Base1:
    x = "base1"

class Base2:
    x = "base2"

class Derived(Base1, Base2):
    pass

print(Derived.x)      # "base1"
print(Derived.mro())  # [__main__.Derived, __main__.Base1, __main__.Base2, object]

If we call the mro() method on the class, it will return a list and the lookup will start from the first element of the list and will end at the last one. Note that the last one element would be object because everything is an object in Python.

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