Inheritance

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Inheritance is an OOP idea that specializes a class based on a general class. The specialized class inherits the attributes from the general class but has some special-case behavior. The general class is called the superclass (or parent class or base class), while the specialized class is called the subclass (or child class).

Motivation

The concept of inheritance stems from the desire to reduce redundancy. Any class that has the same functionality and attributes as a more general class should inherit those traits, as opposed to creating them again. For instance, the Boeing_747 class below should inherit from the Plane class, because the Boeing_747 class shares much functionality with Plane.

class Plane:                                 class Boeing_747:                                         class Boeing_747(Plane): # Boeing_747(Plane) indicates that
                                                                                                                                # Boeing_747 inherits from Plane
    def __init__(self, num_passengers):          def __init__(self, num_first_class, num_economy):         def __init__(self, num_first_class, num_economy):
        ...                                          ...                                                     ...
 
    def fly(self)                                def fly(self):
        ...                                          ...                                                     
 
    def refuel(self)                             def refuel(self):                                        
        ...                                          ...


In the first Boeing_747 class, Plane and Boeing_747 exist as separate classes, each with their own functions. Because they share fly and refuel, Boeing_747 can inherit from Plane and not have to list out the overlapping functions. In the second example, Boeing_747 is a subclass of Plane.

Inheritance allows users to create branching, related systems of classes instead of individual static classes. Because inherited functions and variables are controlled from the super class, the user simply needs to change the inherited values and they will be reflected in all subclasses.

Inheritance vs. composition

Inheritance represents an "is-a" relationship between classes. For example, a Rectangle is a Shape, and a Square is a Rectangle:

class Shape:
    def __init__(self, type):
        self.type = type
 
class Rectangle(Shape):
    def __init__(self, length, width):
        super().__init__("rectangle")
        self.length = length
        self.width = width
 
class Square(Rectangle):
    def __init__(self, side):
	super().__init__(side, side)
        Shape.__init__(self, "square")

Shape is the superclass of Rectangle, and Rectangle is the superclass of Square. A Shape has a type, which is set to "rectangle" and "square" for Rectangle and Square, respectively. A Rectangle has its own attributes, length and width. Square inherits all its attributes from Rectangle and sets its own type.

Composition represents a "has-a" relationship between classes and objects. For example, a Line has endPoints:

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
 
class Line:
    def __init__(self, point1, point2):
        self.start = point1
        self.end = point2

Function lookup

A subclass that doesn't include a certain function, can lookup the function in its super class.

class Plane:                               class Boeing_747(Plane): 
    def __init__(self, num_Passengers):        def __init__(self, num_First_Class num_Economy):
        self.p = num_Passengers                    self.f = num_First_Class
        self.fuel = 0                              self.e = num_Economy
                                                   self.fuel = 0
 
    def fly(self):                            
        print("Takeoff!")                          
 
    def refuel(self):                          
        self.fuel += 100

The Boeing_747 class does not have fly or refuel, but can call them in its superclass.

>>> wikiPlane = Boeing_747(25, 200)
>>> wikiPlane.fly()
Takeoff!
>>> wikiPlane.refuel()
>>> wikiPlane.fuel
100

Here a Boeing_747 object is created, and the user has the Boeing_747 object call fly and refuel. Although the class Boeing_747 does not define fly and refuel, it looks into its super class Plane and calls the functions defined there.

Function and attribute overriding

In the case that a subclass wants to alter a function in its superclass, a subclass can override a superclass's function. The subclass can override a superclass function by defining a function with the exact same name and input arguments as the superclass function. It can then define the function differently from its superclass implementation. The subclass can build on the superclass function as well. A subclass can call a superclass function by invoking the superclass name on the superclass function.

class Plane:                               class Boeing_747(Plane):                                                            class Airforce_One(Boeing_747):
    def __init__(self, num_Passengers):        def __init__(self, num_First_Class num_Economy):                                    def __init__(self, president):
        self.p = num_Passengers                    self.f = num_First_Class                                                            self.VIP = president
        self.fuel = 0                              self.e = num_Economy                                                                self.fuel = 100  
                                                  self.fuel = 0    
 
    def fly(self):                             def fly(self):                                                                      def fly(self): 
        print("Takeoff!")                          print("Boeing 747 clear for...")                                                    print("Renegade is secure") 
                                                   Plane.fly(self)                                                                     Boeing_747.fly(self)  
   def refuel(self):                             
        self.fuel += 100                           #Boeing_747's version of fly() prints the string "Boeing 747 clear for...",         #Airforce_Ones's version of fly() prints the string "Renegade is secure", 
                                                   #then calls Plane's fly() function                                                  #then calls Boeing_747's fly() function
 
 
 
>>> wikiPlane = Boeing_747(25, 200)
>>> wikiPlane.fly()
Boeing 747 clear for...   
Takeoff!   
>>>
>>>ObamasRide = Airforce_One("Obama")
>>>ObamasRide.fly()
Renegade is secure
Boeing 747 clear for...   
Takeoff!

Class variables can similarily be overriden. In the subclass, simply define a class variable with the same name as the one in the superclass to override it.

Multiple inheritance

A class may inherit from multiple superclasses. The method resolution order (MRO) is how Python decides where to look for an attribute of a class that inherits from several classes. For example, we define the following classes:

class A:
    def me(self):
        print("Called A.me")
 
class B:
    def me(self):
        print("Called B.me")
 
class C(A, B):
    pass

C inherits from both A and B. We call C().me() — since C does not define a method me, Python checks the MRO:

>>> C.__mro__
(<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <class 'object'>)

Python first looks in C. Since C doesn't define a method me, Python continues in the MRO and sees that A defines a method me. Python ends up calling me in A:

>>> C().me()
Called A.me


Sources