Python Classes and Objects

Python is an amazing programming language that supports both the functional programming paradigm and object-oriented programming paradigm.
Python's object-oriented programming system supports all the four fundamental features of a general OOPS framework: encapsulation, abstraction, inheritance and polymorphism.

What is a Class and an Object?

• A class is a bundle of instance variables and related methods meant for defining a type of object viewed as a blueprint or a template of the objects.
• An object is an instance of a class with a specific set of attributes.

Class Definition Syntax

To create a class, use the keyword class as shown below

class ClassName:
    <statement-1>

    <statement-N>

Object Types

1.Class Objects
From python documentation;
Class objects support two kinds of operations: attribute references and instantiation.
Attribute references use the standard syntax used for all attribute references in Python: obj.name.

• Valid attribute names are all the names that were in the class’s namespace when the class object was created. Example:

class Student:
    """A simple example class"""
    rollno = 12
    name = "Korir"
def messg(self):
        return 'New Session will start soon.'

• From the above example, Student.roll_no, Student.name are valid attribute references, returning an 12 and 'Korir' respectively.
  Student.messg returns a function object.

• In Python self is a name for the first argument of a method which is different from ordinary function. Rather than passing the object as a parameter in a method the word self refers to the object itself.

Instantiation uses function notation.

To create an instance of a class, you call the class as if it were a function. The example below creates a new instance of the class and assigns this object to the local variable x.

x = Student()

• The instantiation operation creates an empty object. Therefore a class may define a special method named init(), like this:

def __init__(self):
    self.data = []

__ init __() method may have arguments for greater flexibility for example:

class Student:
    """A simple example class"""
    def __init__(self,sroll, sname):
        self.r = sroll
        self.n = sname
    x = Student(10, 'Korir')
 x.r, x.n

Output

(10, 'Korir')

2.Instance Objects

• The only operations understood by instance objects are attribute references. There are two kinds of valid attribute names: data attributes and methods.

Data attributes correspond to instance variables and need not be declared; like local variables, they spring into existence when they are first assigned to.

A method is a function that belongs to an object.
Valid method names of an instance object depend on its class. By definition, all attributes of a class that are function objects define corresponding methods of its instances.

From our example;
x.messg is a valid method reference, since Student.messg is a function.

3.Method Objects

• Usually, a method is called right after it is bound:

x.messg()

• In the Student example, this will return the string 'New Session will start soon'.

Class and Instance Attributes

Instance attribute

• An instance attribute is a Python variable belonging to only one object.
• It is only accessible in the scope of the object and it is defined inside the constructor function of a class. For example:

class Circle:
    def __ init __(self, radius):
        self.pi = 3.14159
        self.radius = radius

    def area(self):
        return self.pi * self.radius**2

    def circumference(self):
        return 2*self.pi * self.radius

• Both pi and radius are called instance attributes. Since they belong to a specific instance of the Circle class.

Class Attribute

• A class attribute is a Python Variable that belongs to a class rather than a particular object.
• It is shared between all other objects of the same class and is defined outside the constructor function __ init __(self,…), of the class. Example:

class Circle:
    pi = 3.14159

    def __init__(self, radius):
        self.radius = radius

    def area(self):
        return self.pi * self.radius**2

    def circumference(self):
        return 2 * self.pi * self.radius

• The above example defines pi as a class attribute
• You can access the class attribute via instances of the class or via the class name:

object_name.class_attribute
class_name.class_attribute

From class attribute example;

c = Circle(10)
print(c.pi)
print(Circle.pi)

Output will be;

3.14159
3.14159

Inheritance

• Like other object-oriented language, Python allows inheritance from a parent (or base) class as well as multiple inheritances in which a class inherits attributes and methods from more than one parent.
• The concept of inheritance provides an important feature to the object-oriented programming is reuse of code
• The syntax for single inheritance is:

class DerivedClassName(BaseClassName):
    <statement-1>

    <statement-N>

• Below is a simple example of single inheritance in python.

class Parent():
       def first(self):
           print('first function')

class Child(Parent):
       def second(self):
          print('second function')

ob = Child()
ob.first()
ob.second()

Here's the output:

first function
second function

• From the above code, you can access the parent class function using the child class object.

Sub-classing

• Calling a constructor of the parent class by mentioning the parent class name in the declaration of the child class is known as sub-classing. A child class identifies its parent class by sub-classing.

• Other types of inheritance are;

Multiple Inheritance-When a child class inherits from more than one parent class.
Multilevel Inheritance-When a child class becomes a parent class for another child class.
Hybrid Inheritance- involves multiple inheritance from the same base or parent class.
Hierarchical Inheritance-involves multiple inheritance taking place in a single program.