4. Understanding self

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What is self?

self is a reference to the current object (instance) of a class.

When you call a method using an object, Python automatically passes that object as the first argument to the method.

Example

class Student:

    def show(self):
        print(self)

s1 = Student()

s1.show()

Output

<__main__.Student object at 0x123456>

Key Point

self is the object that called the method.

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What Python Does Internally

When you write:

s1.show()

Python internally does something similar to:

Student.show(s1)

The object (s1) is automatically passed as the first argument.

Therefore:

def show(self):

becomes:

def show(s1):

for that particular method call.

Notes

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Why Do We Need self?

Suppose we create two objects.

class Student:

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

s1 = Student("John")
s2 = Student("Alice")

Python stores data separately for each object.

Memory Representation

s1
 └── name = John

s2
 └── name = Alice

Key Point

self tells Python which object's data should be used.

Notes

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How self Works Inside __init__

Example:

class Student:

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

When:

s1 = Student("John")

Python internally performs something similar to:

Student.__init__(s1, "John")

Inside the method:

self = s1
name = "John"

Then:

self.name = name

becomes:

s1.name = "John"

Key Point

self.name stores data inside the object.

Notes

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Visual Representation

Creating first object:

s1 = Student("John")

Memory:

s1
│
├── name = John

Creating second object:

s2 = Student("Alice")

Memory:

s1
│
├── name = John

s2
│
├── name = Alice

Key Point

Each object has its own copy of instance variables.

Notes

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Accessing Variables Using self

Example:

class Student:

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

    def display(self):
        print(self.name)

s1 = Student("John")

s1.display()

Output

John

Explanation

Inside display():

self.name

means:

s1.name

because self refers to the object that called the method.

Notes

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Accessing Methods Using self

Methods inside a class can call other methods using self.

Example

class Student:

    def greet(self):
        print("Hello")

    def display(self):
        self.greet()
        print("Welcome")

s1 = Student()

s1.display()

Output

Hello
Welcome

Explanation

self.greet()

internally becomes:

Student.greet(self)

If self refers to s1:

Student.greet(s1)

Key Point

Use self.method_name() to call another method of the same object.

Notes

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What Happens If We Don't Use self?

Incorrect Example

class Student:

    def __init__(self, name):
        name = name

Problem

This only creates a local variable.

After the constructor finishes, the variable disappears.

Nothing is stored inside the object.

Notes

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Correct Usage

class Student:

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

Now the value is stored inside the object.

s1 = Student("John")

print(s1.name)

Output

John

Notes

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Most Important Rule

Whenever data should belong to an object:

self.variable_name

Examples:

self.name
self.age
self.salary
self.balance

Whenever you want to call another method of the same object:

self.method_name()

Examples:

self.display()
self.save()
self.calculate()

Notes

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Real-Life Analogy

Imagine:

s1 = Student("John")
s2 = Student("Alice")

Think of self as asking:

"Which student am I currently talking about?"

For s1:

self.name

means:

John

For s2:

self.name

means:

Alice

The same code works for every object because self always points to the current object.

Notes

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Interview Question

Q: Is self a keyword in Python?

Answer: No.

You can technically use another name.

Example:

class Student:

    def show(my_object):
        print(my_object)

Python will still work.

However, using self is the official convention and should always be followed.

Recommended Style

class Student:

    def show(self):
        print(self)

Notes

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Summary

• self refers to the current object.
• Python automatically passes the object as the first argument.
• Use self.variable to store and access object data.
• Use self.method() to call another method in the same object.
• Without self, values are local variables and disappear after method execution.
• self is not a Python keyword, but it is the standard convention.

Revision Checklist

• What is self?
• How Python passes self
• Accessing variables using self
• Calling methods using self
• Why self.variable is required
• Interview question on self