Python OOPs Concepts

14 Jun, 2022
In Python, object-oriented Programming (OOPs) is a programming paradigm that uses objects and classes in programming. It aims to implement real-world entities like inheritance, polymorphisms, encapsulation, etc. in the programming. The main concept of OOPs is to bind the data and the functions that work on that together as a single unit so that no other part of the code can access this data.
Class
Objects
Polymorphism
Encapsulation
Inheritance
Data Abstraction
OOPs 
A class is a collection of objects. A class contains the blueprints or the prototype from which the objects are being created. It is a logical entity that contains some attributes and methods. 
To understand the need for creating a class let’s consider an example, let’s say you wanted to track the number of dogs that may have different attributes like breed, age. If a list is used, the first element could be the dog’s breed while the second element could represent its age. Let’s suppose there are 100 different dogs, then how would you know which element is supposed to be which? What if you wanted to add other properties to these dogs? This lacks organization and it’s the exact need for classes. 
Some points on Python class:  
Classes are created by keyword class.
Attributes are the variables that belong to a class.
Attributes are always public and can be accessed using the dot (.) operator. Eg.: Myclass.Myattribute
Class Definition Syntax:
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class ClassName:
   # Statement-1
   .
   .
   .
   # Statement-N
Python
# Python3 program to# demonstrate defining# a class class Dog:    pass
In the above example, we have created a class named dog using the class keyword.
The object is an entity that has a state and behavior associated with it. It may be any real-world object like a mouse, keyboard, chair, table, pen, etc. Integers, strings, floating-point numbers, even arrays, and dictionaries, are all objects. More specifically, any single integer or any single string is an object. The number 12 is an object, the string “Hello, world” is an object, a list is an object that can hold other objects, and so on. You’ve been using objects all along and may not even realize it.
An object consists of :
State: It is represented by the attributes of an object. It also reflects the properties of an object.
Behavior: It is represented by the methods of an object. It also reflects the response of an object to other objects.
Identity: It gives a unique name to an object and enables one object to interact with other objects.
To understand the state, behavior, and identity let us take the example of the class dog (explained above). 
The identity can be considered as the name of the dog.
State or Attributes can be considered as the breed, age, or color of the dog.
The behavior can be considered as to whether the dog is eating or sleeping.
Python3
obj = Dog()
This will create an object named obj of the class Dog defined above. Before diving deep into objects and class let us understand some basic keywords that will we used while working with objects and classes.
Class methods must have an extra first parameter in the method definition. We do not give a value for this parameter when we call the method, Python provides itIf we have a method that takes no arguments, then we still have to have one argument.This is similar to this pointer in C++ and this reference in Java.
Class methods must have an extra first parameter in the method definition. We do not give a value for this parameter when we call the method, Python provides it
If we have a method that takes no arguments, then we still have to have one argument.
This is similar to this pointer in C++ and this reference in Java.
When we call a method of this object as myobject.method(arg1, arg2), this is automatically converted by Python into MyClass.method(myobject, arg1, arg2) – this is all the special self is about.
Note: For more information, refer to self in Python class
The __init__ method is similar to constructors in C++ and Java. It is run as soon as an object of a class is instantiated. The method is useful to do any initialization you want to do with your object. 
Now let us define a class and create some objects using the self and __init__ method.
Python3
class Dog:     # class attribute    attr1 = "mammal"     # Instance attribute    def __init__(self, name):        self.name = name # Driver code# Object instantiationRodger = Dog("Rodger")Tommy = Dog("Tommy") # Accessing class attributesprint("Rodger is a {}".format(Rodger.__class__.attr1))print("Tommy is also a {}".format(Tommy.__class__.attr1)) # Accessing instance attributesprint("My name is {}".format(Rodger.name))print("My name is {}".format(Tommy.name))
Rodger is a mammal
Tommy is also a mammal
My name is Rodger
My name is Tommy
Python3
class Dog:     # class attribute    attr1 = "mammal"     # Instance attribute    def __init__(self, name):        self.name = name             def speak(self):        print("My name is {}".format(self.name)) # Driver code# Object instantiationRodger = Dog("Rodger")Tommy = Dog("Tommy") # Accessing class methodsRodger.speak()Tommy.speak()
My name is Rodger
My name is Tommy
Note: For more information, refer Python Classes and Objects
Inheritance is the capability of one class to derive or inherit the properties from another class. The class that derives properties is called the derived class or child class and the class from which the properties are being derived is called the base class or parent class. The benefits of inheritance are:
It represents real-world relationships well.
It provides the reusability of a code. We don’t have to write the same code again and again. Also, it allows us to add more features to a class without modifying it.
It is transitive in nature, which means that if class B inherits from another class A, then all the subclasses of B would automatically inherit from class A.
Single Inheritance:Single-level inheritance enables a derived class to inherit characteristics from a single-parent class.
Multilevel Inheritance:Multi-level inheritance enables a derived class to inherit properties from an immediate parent class which in turn inherits properties from his parent class.
Hierarchical Inheritance:Hierarchical level inheritance enables more than one derived class to inherit properties from a parent class.
Multiple Inheritance:Multiple level inheritance enables one derived class to inherit properties from more than one base class.
Python3
# Python code to demonstrate how parent constructors# are called. # parent classclass Person(object):     # __init__ is known as the constructor    def __init__(self, name, idnumber):        self.name = name        self.idnumber = idnumber     def display(self):        print(self.name)        print(self.idnumber)             def details(self):        print("My name is {}".format(self.name))        print("IdNumber: {}".format(self.idnumber))     # child classclass Employee(Person):    def __init__(self, name, idnumber, salary, post):        self.salary = salary        self.post = post         # invoking the __init__ of the parent class        Person.__init__(self, name, idnumber)             def details(self):        print("My name is {}".format(self.name))        print("IdNumber: {}".format(self.idnumber))        print("Post: {}".format(self.post))  # creation of an object variable or an instancea = Employee('Rahul', 886012, 200000, "Intern") # calling a function of the class Person using# its instancea.display()a.details()
Rahul
886012
My name is Rahul
IdNumber: 886012
Post: Intern
In the above article, we have created two classes i.e. Person (parent class) and Employee (Child Class). The Employee class inherits from the Person class. We can use the methods of the person class through employee class as seen in the display function in the above code. A child class can also modify the behavior of the parent class as seen through the details() method.
Note: For more information, refer to our Inheritance in Python tutorial.
Polymorphism simply means having many forms. For example, we need to determine if the given species of birds fly or not, using polymorphism we can do this using a single function.
Python3
class Bird:       def intro(self):        print("There are many types of birds.")     def flight(self):        print("Most of the birds can fly but some cannot.") class sparrow(Bird):       def flight(self):        print("Sparrows can fly.") class ostrich(Bird):     def flight(self):        print("Ostriches cannot fly.") obj_bird = Bird()obj_spr = sparrow()obj_ost = ostrich() obj_bird.intro()obj_bird.flight() obj_spr.intro()obj_spr.flight() obj_ost.intro()obj_ost.flight()
There are many types of birds.
Most of the birds can fly but some cannot.
There are many types of birds.
Sparrows can fly.
There are many types of birds.
Ostriches cannot fly.
Note: For more information, refer to our Polymorphism in Python Tutorial.
Encapsulation is one of the fundamental concepts in object-oriented programming (OOP). It describes the idea of wrapping data and the methods that work on data within one unit. This puts restrictions on accessing variables and methods directly and can prevent the accidental modification of data. To prevent accidental change, an object’s variable can only be changed by an object’s method. Those types of variables are known as private variables.
A class is an example of encapsulation as it encapsulates all the data that is member functions, variables, etc.
Python3
# Python program to# demonstrate private members # Creating a Base classclass Base:    def __init__(self):        self.a = "GeeksforGeeks"        self.__c = "GeeksforGeeks" # Creating a derived classclass Derived(Base):    def __init__(self):         # Calling constructor of        # Base class        Base.__init__(self)        print("Calling private member of base class: ")        print(self.__c)  # Driver codeobj1 = Base()print(obj1.a) # Uncommenting print(obj1.c) will# raise an AttributeError # Uncommenting obj2 = Derived() will# also raise an AtrributeError as# private member of base class# is called inside derived class
GeeksforGeeks
In the above example, we have created the c variable as the private attribute. We cannot even access this attribute directly and can’t even change its value.
Note: for more information, refer to our Encapsulation in Python Tutorial.
It hides the unnecessary code details from the user. Also,  when we do not want to give out sensitive parts of our code implementation and this is where data abstraction came.
Data Abstraction in Python can be achieved through creating abstract classes.
Object Oriented Programming in Python | Set 2 (Data Hiding and Object Printing)
https://youtu.be/CiH7lN4
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