The set of all the values or contents of a class object’s instance variables 2024

Xem The set of all the values or contents of a class object’s instance variables 2024

In Object-oriented programming, when we design a class, we use instance variables and class variables.

  • Instance variables: If the value of a variable varies from object to object, then such variables are called instance variables.
  • Class Variables: A class variable is a variable that is declared inside of class, but outside of any instance method or
    class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

    1 method.

After reading this article, you’ll learn:

  • How to create and access instance variables
  • Modify values of instance variables
  • How to dynamically add or delete instance variables
  • Scope of a instance variables

Table of contents

What is an Instance Variable in Python?

If the value of a variable varies from object to object, then such variables are called instance variables. For every object, a separate copy of the instance variable will be created.

Instance variables are not shared by objects. Every object has its own copy of the instance attribute. This means that for each object of a class, the instance variable value is different.

When we create classes in Python, instance methods are used regularly. we need to create an object to execute the block of code or action defined in the instance method.

Instance variables are used within the instance method. We use the instance method to perform a set of actions on the data/value provided by the instance variable.

We can access the instance variable using the object and dot (

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

2) operator.

In Python, to work with an instance variable and method, we use the 

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

3 keyword. We use the 

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

3 keyword as the first parameter to a method. The

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

3 refers to the current object.

Declare Instance variable

Create Instance Variables

Instance variables are declared inside a method using the

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

3 keyword. We use a constructor to define and initialize the instance variables. Let’s see the example to declare an instance variable in Python.

Example:

In the following example, we are creating two instance variable

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

7 and

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

8 in the

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

9 class.

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create first object
s1 = Student("Jessa", 20)

# access instance variable
print('Object 1')
print('Name:', s1.name)
print('Age:', s1.age)

# create second object
s2= Student("Kelly", 10)

# access instance variable
print('Object 2')
print('Name:', s2.name)
print('Age:', s2.age)

Output

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

Note:

  • When we created an object, we passed the values to the instance variables using a constructor.
  • Each object contains different values because we passed different values to a constructor to initialize the object.
  • Variable declared outside
    class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

    1 belong to the class. They’re shared by all instances.

Modify Values of Instance Variables

We can modify the value of the instance variable and assign a new value to it using the object reference.

Note: When you change the instance variable’s values of one object, the changes will not be reflected in the remaining objects because every object maintains a separate copy of the instance variable.

Example

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

Output

Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

Ways to Access Instance Variable

There are two ways to access the instance variable of class:

  • Within the class in instance method by using the object reference (
    class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

print('Before')
print('Name:', stud.name, 'Age:', stud.age)

# modify instance variable
stud.name = 'Emma'
stud.age = 15

print('After')
print('Name:', stud.name, 'Age:', stud.age)

    3)

  • Using
    Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

    2 method

Example 1: Access instance variable in the instance method

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

    # instance method access instance variable
    def show(self):
        print('Name:', stud.name, 'Age:', stud.age)

# create object
stud = Student("Jessa", 20)

# call instance method
stud.show()

Output

Name: Jessa Age: 20

instance variables and methods

Example 2: Access instance variable using

Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

2

getattr(Object, 'instance_variable')

Pass the object reference and instance variable name to the

Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

2 method to get the value of an instance variable.

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

# create object
stud = Student("Jessa", 20)

# Use getattr instead of stud.name
print('Name:', getattr(stud, 'name'))
print('Age:', getattr(stud, 'age'))

Output

Name: Jessa
Age: 20

Instance Variables Naming Conventions

  • Instance variable names should be all lower case. For example,
    Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

    5

  • Words in an instance variable name should be separated by an underscore. For example,
    Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

    6

  • Non-public instance variables should begin with a single underscore
  • If an instance name needs to be mangled, two underscores may begin its name

Dynamically Add Instance Variable to a Object

We can add instance variables from the outside of class to a particular object. Use the following syntax to add the new instance variable to the object.

object_referance.variable_name = value

Example:

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

0

Output

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

1

Note:

  • We cannot add an instance variable to a class from outside because instance variables belong to objects.
  • Adding an instance variable to one object will not be reflected the remaining objects because every object has a separate copy of the instance variable.

Dynamically Delete Instance Variable

In Python, we use the

Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

7 statement and

Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

8 function to delete the attribute of an object. Both of them do the same thing.

  • Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

    7 statement: The

    Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

    7 keyword is used to delete objects. In Python, everything is an object, so the

    Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

    7 keyword can also be used to delete variables, lists, or parts of a list, etc.

  • Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

    8 function: Used to delete an instance variable dynamically.

Note: When we try to access the deleted attribute, it raises an attribute error.

Example 1: Using the

Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

7 statement

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

2

Output

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

3

Before
Name: Jessa Age: 20

After
Name: Emma Age: 15

8 function

The delattr() function is used to delete the named attribute from the object with the prior permission of the object. Use the following syntax.

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

4

  • class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

    # instance method access instance variable
    def show(self):
        print('Name:', stud.name, 'Age:', stud.age)

# create object
stud = Student("Jessa", 20)

# call instance method
stud.show()

    5: the object whose attribute we want to delete.

  • class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

    # instance method access instance variable
    def show(self):
        print('Name:', stud.name, 'Age:', stud.age)

# create object
stud = Student("Jessa", 20)

# call instance method
stud.show()

    6: the name of the instance variable we want to delete from the object.

Example

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

5

Output

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

6

Access Instance Variable From Another Class

We can access instance variables of one class from another class using object reference. It is useful when we implement the concept of inheritance in Python, and we want to access the parent class instance variable from a child class.

let’s understand this with the help of an example.

In this example, the

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

    # instance method access instance variable
    def show(self):
        print('Name:', stud.name, 'Age:', stud.age)

# create object
stud = Student("Jessa", 20)

# call instance method
stud.show()

7 is an instance variable of the

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

    # instance method access instance variable
    def show(self):
        print('Name:', stud.name, 'Age:', stud.age)

# create object
stud = Student("Jessa", 20)

# call instance method
stud.show()

8 class. We inherited a

class Student:
    # constructor
    def __init__(self, name, age):
        # Instance variable
        self.name = name
        self.age = age

    # instance method access instance variable
    def show(self):
        print('Name:', stud.name, 'Age:', stud.age)

# create object
stud = Student("Jessa", 20)

# call instance method
stud.show()

8 class to access its instance variables in

Name: Jessa Age: 20

0 class

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

7

Output

Object 1
Name: Jessa
Age: 20

Object 2
Name: Kelly
Age: 10

8

List all Instance Variables of a Object

We can get the list of all the instance variables the object has. Use the

Name: Jessa Age: 20

1 function of an object to get all instance variables along with their value.

The

Name: Jessa Age: 20

1 function returns a dictionary that contains variable name as a key and variable value as a value

What is the instance of a class called?

An instance of a class is an object. It is also known as a class object or class instance. As such, instantiation may be referred to as construction.

What is an object instance of a class?

an object is an element (or instance) of a class; objects have the behaviors of their class. The object is the actual component of programs, while the class specifies how instances are created and how they behave.

What are class object variables?

An object variable or instance member belongs to a specific instance of a class. That is to say that every instance has its own copy of that piece of data. A class variable or static member is shared by every instance of the class.

What is an instance variable in Java?

An instance variable is a variable that is specific to a certain object. It is declared within the curly braces of the class but outside of any method. The value of an instance variable can be changed by any method in the class, but it is not accessible from outside the class.

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