Merge pull request #22 from basnetsoyuj/master

Added 22-inheritance.md

Author: parewa-ranjit

22-inheritance.md

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+# Python Inheritance
+
+**Video link:** [https://youtu.be/C8qE3mKiBrQ](https://youtu.be/C8qE3mKiBrQ)
+
+In this video, you learned about inheritance in Python with the help of examples. We also covered the concepts of method overriding and the `super()` function.
+
+**Programs in the Video**
+
+- [Inheritance in Python](#inheritance-in-python)
+- [Example of Inheritance](#example-of-inheritance)
+- [Method Overriding](#method-overriding)
+- [The `super()` function](#the-super-function)
+
+---
+
+## Inheritance in Python
+Let's derive a dog and cat class from an animal class and get a feel of how inheritance works.
+
+Suppose we have an `Animal` class:
+
+```python
+class Animal:
+    def eat(self):
+        print("I can eat")
+```
+
+Now, let's derive a `Dog` class from `Animal`:
+
+```python
+class Animal:
+    def eat():
+        print("I can eat")
+        
+class Dog(Animal):
+    def bark(self):
+        print("I can bark")
+```
+
+Objects of `Dog` can access methods and attributes of both `Dog` and `Animal`:
+
+
+```python
+class Animal:
+    def eat():
+        print("I can eat")
+        
+class Dog(Animal):
+    def bark(self):
+        print("I can bark")
+
+dog1 = Dog()
+
+dog1.bark()
+dog1.eat()
+```
+
+**Output**
+```
+I can bark
+I can eat
+```
+
+Similarly, if we want, we can derive another `Cat` class from the `Animal` class:
+
+
+```python
+class Animal:
+    def eat():
+        print("I can eat")
+        
+class Dog(Animal):
+    def bark(self):
+        print("I can bark")
+
+class Cat(Animal):
+    def get_grumpy(self):
+        print("I am getting grumpy.")
+
+dog1 = Dog()
+
+dog1.bark()
+dog1.eat()
+
+cat1 = Cat()
+cat1.eat()
+```
+
+**Output**
+
+```
+I can bark
+I can eat
+I can eat
+```
+
+---
+
+## Example of Inheritance
+
+Let's implement a program to calculate the perimeter of different polygons like triangles and quadrilaterals using inheritance.
+
+Let's first create a base class called `Polygon`:
+
+```python
+class Polygon:
+    def __init__(self, sides):
+        self.sides = sides
+
+    def display_info(self):
+        print("A polygon is a two dimensional shape with straight lines")
+
+    def get_perimeter(self):
+        perimeter = sum(self.sides)
+        return perimeter
+```
+
+All polygons like triangles and quadrilaterals will derive these features.
+
+Let's create a `Triangle` class that will inherit from the `Polygon` class:
+
+
+```python
+class Polygon:
+    def __init__(self, sides):
+        self.sides = sides
+
+    def display_info(self):
+        print("A polygon is a two dimensional shape with straight lines")
+
+    def get_perimeter(self):
+        perimeter = sum(self.sides)
+        return perimeter
+
+
+class Triangle(Polygon):
+    def display_info(self):
+        print("A triangle is a polygon with 3 edges")
+```
+
+I have redefined the `display_info()` method to display information specific to triangles.
+
+Similarly, I will also define a new `Quadrilateral` class.
+
+```python
+class Polygon:
+    def __init__(self, sides):
+        self.sides = sides
+
+    def display_info(self):
+        print("A polygon is a two dimensional shape with straight lines")
+
+    def get_perimeter(self):
+        perimeter = sum(self.sides)
+        return perimeter
+
+
+class Triangle(Polygon):
+    def display_info(self):
+        print("A triangle is a polygon with 3 edges")
+
+
+class Quadrilateral(Polygon):
+    def display_info(self):
+        print("A quadrilateral is a polygon with 4 edges")
+```
+
+Now, let's find the perimeter of a triangle. I will create an object from `Triangle` and use its `get_perimeter()` method.
+
+
+```python
+class Polygon:
+    def __init__(self, sides):
+        self.sides = sides
+
+    def display_info(self):
+        print("A polygon is a two dimensional shape with straight lines")
+
+    def get_perimeter(self):
+        value = 0
+        for side in self.sides:
+            value += side
+        return value
+
+
+class Triangle(Polygon):
+    def display_info(self):
+        print("A triangle is a polygon with 3 edges")
+
+
+class Quadrilateral(Polygon):
+    def display_info(self):
+        print("A quadrilateral is a polygon with 4 edges")
+
+t1 = Triangle([5, 6, 7])
+perimeter = t1.get_perimeter()
+print("Perimeter:", perimeter)
+```
+
+**Output**
+
+```
+Perimeter: 18
+```
+
+---
+
+## Method Overriding
+
+If you have noticed, in the above example, we have the `display_info()` method in both our base class and the derived classes.
+
+Let's see what will happen if we call the `display_info()` method for the `t1` triangle.
+
+```python
+class Polygon:
+    def __init__(self, sides):
+        self.sides = sides
+
+    def display_info(self):
+        print("A polygon is a two dimensional shape with straight lines")
+
+    def get_perimeter(self):
+        perimeter = sum(self.sides)
+        return perimeter
+
+
+class Triangle(Polygon):
+    def display_info(self):
+        print("A triangle is a polygon with 3 edges")
+
+
+class Quadrilateral(Polygon):
+    def display_info(self):
+        print("A quadrilateral is a polygon with 4 edges")
+
+t1 = Triangle([5, 6, 7])
+perimeter = t1.get_perimeter()
+print("Perimeter:", perimeter)
+
+t1.display_info()
+```
+
+**Output**
+
+```
+Perimeter: 18
+A triangle is a polygon with 3 edges
+```
+
+We can see that the `display_info()` method of the `Triangle` class is called and `display_info()` of its parent class is not executed.
+
+This is called method overriding.
+
+**If the same method is defined in both the base and the derived class, then the method of the derived class overrides the method of the base class.**
+
+
+If we need, we can call the `display_info()` method of our parent `Polygon` class from inside its child classes like this: 
+
+```python
+class Polygon:
+    def __init__(self, sides):
+        self.sides = sides
+
+    def display_info(self):
+        print("A polygon is a two dimensional shape with straight lines")
+
+    def get_perimeter(self):
+        perimeter = sum(self.sides)
+        return perimeter
+
+
+class Triangle(Polygon):
+    def display_info(self):
+        print("A triangle is a polygon with 3 edges")
+        Polygon.display_info(self)
+
+
+class Quadrilateral(Polygon):
+    def display_info(self):
+        print("A quadrilateral is a polygon with 4 edges")
+
+t1 = Triangle([5, 6, 7])
+perimeter = t1.get_perimeter()
+print("Perimeter:", perimeter)
+
+t1.display_info()
+```
+
+**Output**
+```
+Parameter: 18
+A triangle is a polygon with 3 edges
+A polygon is a two dimensional shape with straight lines
+```
+
+>**Note:** `Polygon` is the name of the parent class. Since we are calling the method using a class rather than an object, we also need to pass the `self` object manually.
+
+This code is a bit more unorthodox than what we have been using. There is a more elegant way to achieve the same task by using the `super()` function.
+
+---
+
+## The `super()` function
+
+The `super()` function returns a temporary object of the superclass for a subclass.
+
+```python 
+class Polygon:
+    def __init__(self, sides):
+        self.sides = sides
+
+    def display_info(self):
+        print("A polygon is a two dimensional shape with straight lines")
+
+    def get_perimeter(self):
+        value = 0
+        for side in self.sides:
+            value += side
+        return value
+
+
+class Triangle(Polygon):
+    def display_info(self):
+        print("A triangle is a polygon with 3 edges")
+        # Polygon.display_info(self)
+        super().display_info()
+
+class Quadrilateral(Polygon):
+    def display_info(self):
+        print("A quadrilateral is a polygon with 4 edges")
+
+t1 = Triangle([5, 6, 7])
+perimeter = t1.get_perimeter()
+print("Perimeter:", perimeter)
+
+t1.display_info()
+```
+
+**Output**
+
+```
+Perimeter: 18
+A triangle is a polygon with 3 edges
+A polygon is a two dimensional shape with straight lines
+```
+
+In this case, `super()` is an object of `Polygon`. We are using it to call `display_info()` of the `Polygon` class.