Objects

The object is an entity that has a state and behavior associated with it. It may be any real world entity like: mouse, keyboard, pen, table, chair, etc.

Object consist 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 reflect 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 dog:-

• 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 the whether the dog is eating or sleeping.

Creating an object: obj = Dog()

Python Self

• Class methods must have an extra 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 argument, then we will still have to have one argument.
• This is similar to this pointer in C++, and this reference in Java.

Use of self : When working with the classes in python, the term self refers to the instance of the class that is currently being used. It is customary to use "self" as the first parameter in instance methods of a class. Whenever you call a method of an object created from a class, the object is automatically passed as the first argument using the self parameter. This enables you to modify the objects properties and executes takes unique to that particular instance.

Example:

class myNumber:
    def __init__(self, value):
        self.value = value
    def printValue(self):
        print(self.value)

obj1 = myNumber(30)
obj1.printValue()

"""
Output:
30
"""

Self Parameter 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.

class cambridgeCSLectures:
    def __init__(self, name, lecturer, course, year, semester):
        self.name = name
        self.lecturer = lecturer
        self.course = course
        self.year = year
        self.semester = semester
    def displayDetails(self):
        print("Hello, My name is " + self.name + " as a " + self.lecturer + " lecturer of " + self.course + " of " + self.year + " of " + self.semester + " semester ")

obj = cambridgeCSLectures("Vayishu", "Computer Science", "Introduction to Python Programming", "2024", "2nd")
obj.displayDetails()

"""
Output:
Hello, My name is Vayishu as a Computer Science lecturer of Introduction to Python Programming of 2024 of 2nd semester
"""

The self parameter doesn't call it to be self, you can use any other name instead of it.

• __init__() method: This method is called constructors. it is as similar as the C++ and Java constructors. Constructors are used to initializing the objects state. Like methods, a constructor also contains a collections of statements that are executed at the time of the object creation. It runs as soon as the object of a class is instantiated.
• self is not considered as a keyword in python. Self is just a parameter name used in instance methods to refer to the instance itself.

=> The __init__ method is similar to constructor in CPP and Java. It is run as soon as the object os a class is instantiated. The method is useful to do any initialization you want to do with your object.

Class and instance variable or attribute

• Instance variable are for data, unique to each instance and class variable are for attributes and methods shared by all instances of the class.
• Instance variable are variable whose value is assigned inside a constructor or method with self where as class variable are variable whose value is assigned in the class.
• instance variable = variable inside the constructor

class Dog:
    animal = 'dog' # class variable
    def __init__(self, name, breed): # init method or constructor
        self.name = name # instance variable 1
        self.breed = breed # instance variable 2
    def setColor(self, color):
        self.color = color # instance variable 3
    def getDetails(self):
        print("I have my pet and his name is " + self.name + " and he belongs to the breed of " + self.breed) # Retrieves an instance variable 1 and 2
    def getColor(self):
        return self.color # Retrieves an instance variable 3

petDog = Dog("Jimmy", "Pug")
petDog.setColor("White")
petDog.getDetails()
color = petDog.getColor()
print("Color: ", color)

"""
Output:
I have my pet and his name is Jimmy and he belongs to the breed of Pug
Color:  White
"""

Define a class in Python? To define a class in python, use the class keyboard foloowed by the class name and a colon.

class myClass:
    def __init__(self, arg1, arg2):
        self.arg1 = arg1
        self.arg2 = arg2
    def someMethod(self):
        # Method definitions
        pass

Example-1 and Example-2 are in the folder of the ClassWork Programs and into the folder of the Classes and Objects.