Instance Methods¶

What is an Instance Method?¶

An instance method is a method that operates on a specific object (instance) of a class.

It receives the instance as its first parameter, conventionally named self.

Because it has access to the object, an instance method can read and modify that object's state.

A Simple Example¶

class User:

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

user = User()

user.greet()

This looks straightforward.

But something interesting happens here.

The method definition is

def greet(self):

which expects one argument.

Yet we call

user.greet()

without passing anything.

So where did self come from?

What Actually Happens?¶

When Python sees

user.greet()

it automatically passes the instance.

Conceptually,

User.greet(user)

Both are equivalent.

user.greet()

          │

          ▼

User.greet(user)

Python performs this method binding automatically.

Understanding self¶

self is not a keyword.

It is simply the first parameter that receives the current object.

This works too.

class User:

    def greet(me):

        print(me)

Although valid,

using anything other than self is strongly discouraged because self is the accepted Python convention.

Memory Representation¶

                Class

              User

                 │

          greet(self)

                 │


------------------------------------


user

      │

      ▼

+-----------------------+

| name = "Gaurav"       |

| age = 25              |

+-----------------------+

Calling

user.greet()

binds the object to the method.

user

        │

        ▼

greet(user)

Inside the method,

self

        │

        ▼

same object

Both user and self reference the same instance.

Verifying It¶

class User:

    def show(self):

        print(id(self))

user = User()

print(id(user))

user.show()

Output

140238974856400

140238974856400

Both IDs are identical.

This proves that self and user reference the same object.

Why Do We Need self?¶

Suppose a class stores user information.

class User:

    def __init__(self, name):

        self.name = name

    def greet(self):

        print(self.name)

Create two users.

u1 = User("Alice")

u2 = User("Bob")

Calling

u1.greet()

prints

Alice

Calling

u2.greet()

prints

Bob

How does Python know which name to print?

Because self points to the instance that invoked the method.

Conceptually,

u1.greet()

↓

greet(u1)

↓

self = u1

↓

Alice

u2.greet()

↓

greet(u2)

↓

self = u2

↓

Bob

Bound Methods¶

When we access

user.greet

notice that we didn't call the method.

There are no parentheses.

method = user.greet

What is stored inside method?

Let's check.

print(type(method))

Output

<class 'method'>

Python creates a bound method.

A bound method is simply a method that already remembers which object it belongs to.

Conceptually,

user.greet

↓

Bound Method

↓

Function + user

It stores two things.

Bound Method

│

├── Function (greet)

└── Instance (user)

Now calling

method()

is equivalent to

User.greet(user)

The instance has already been remembered.

Bound Method Visualization¶

          user

           │

           ▼

      Bound Method

      ┌──────────────┐

      │ greet()      │

      │ instance=user│

      └──────────────┘

Calling

method()

automatically supplies

self = user

Accessing the Method from the Class¶

Now try

User.greet

Notice the difference.

No object exists.

So Python returns the function itself.

User.greet

↓

Function Object

Calling

User.greet()

raises an error.

TypeError

missing required positional argument 'self'

Because no instance was provided.

Instead,

User.greet(user)

works perfectly.

Instance Methods Can Access Instance Data¶

class User:

    def __init__(self, name):

        self.name = name

    def rename(self, name):

        self.name = name

The method modifies the current object's state.

Before

user

↓

name = Alice


rename("Bob")


After

↓

name = Bob

When Should You Use Instance Methods?¶

Use instance methods whenever the behavior depends on the state of a specific object.

Examples include:

Updating account balance
Changing user profile
Validating object state
Reading instance attributes

Where are instance methods stored?¶

Instance methods are stored on the class, not on each object.

When accessed through an instance, Python creates a bound method that combines the function from the class with the current instance.

          User Class

        greet()

            │

            ▼

        Function Object

            ▲

            │

user.greet

↓

Bound Method

↓

Function + user

This design avoids storing a separate copy of every method for each object, making Python classes memory efficient.

Common Interview Questions¶

What is an instance method?¶

An instance method is a method that operates on a specific object. It automatically receives the instance as its first argument (self), allowing it to access and modify that object's state.

What is `self`?¶

self is a reference to the current instance of the class. Python automatically passes it when an instance method is invoked through an object.

Is `self` a keyword?¶

No. self is just a naming convention. You can technically use any name, but self is the universally accepted convention in Python.

How is `self` passed automatically?¶

When you access a method through an instance, Python creates a bound method. The bound method stores the instance and automatically passes it as the first argument when invoked.

Conceptually,

user.greet()

becomes

User.greet(user)

What is a bound method?¶

A bound method is a method object that remembers the instance it belongs to. When called, Python automatically supplies that stored instance as self.

Why does `User.greet()` raise an error?¶

Because greet() expects an instance as its first argument. Calling it directly from the class doesn't provide self, so Python raises a TypeError.

Correct usage:

User.greet(user)

When should you use an instance method?¶

Use an instance method whenever the behavior depends on the state of a particular object. If the method needs to read or modify instance attributes, it should be an instance method.

Can an instance method access class variables?¶

Yes. An instance method can access both instance variables and class variables through self or the class itself.

Can an instance method be called without creating an object?¶

Yes, but you must explicitly pass an instance.

user = User()

User.greet(user)

This is exactly what Python does internally when you write:

user.greet()

Why are methods defined inside the class instead of the object?¶

Methods represent behavior that is common to all instances of a class. Storing them on the class avoids duplicating the same function for every object, making memory usage more efficient.

Common Interview Mistakes¶

❌ Thinking self is a keyword.

✔ It is simply the first parameter representing the current instance.

❌ Thinking Python creates a copy of the object for self.

✔ self references the exact same object.

❌ Thinking instance methods belong to the object.

✔ Methods are defined on the class.

Objects receive bound methods when accessed through an instance.

Interview Tip¶

If asked,

How does Python automatically pass self?

A strong answer is:

"When an instance method is accessed through an object, Python creates a bound method. The bound method stores both the original function and the instance. When the bound method is invoked, Python automatically passes the stored instance as the first argument (self). Conceptually, obj.method() is equivalent to Class.method(obj)."

Key Takeaways¶

Instance methods operate on a specific object.
self is a reference to the current instance.
self is passed automatically by Python.
obj.method() is conceptually equivalent to Class.method(obj).
Accessing a method through an instance creates a bound method.
A bound method remembers both the function and the instance.
Instance methods are used whenever behavior depends on object state.

Instance Methods¶

What is an Instance Method?¶

A Simple Example¶

What Actually Happens?¶

Understanding self¶

Memory Representation¶

Verifying It¶

Why Do We Need self?¶

Bound Methods¶

Bound Method Visualization¶

Accessing the Method from the Class¶

Instance Methods Can Access Instance Data¶

When Should You Use Instance Methods?¶

Where are instance methods stored?¶

Common Interview Questions¶

What is an instance method?¶

What is self?¶

Is self a keyword?¶

How is self passed automatically?¶

What is a bound method?¶

Why does User.greet() raise an error?¶

When should you use an instance method?¶

Can an instance method access class variables?¶

Can an instance method be called without creating an object?¶

Why are methods defined inside the class instead of the object?¶

Common Interview Mistakes¶

Interview Tip¶

Key Takeaways¶

What is `self`?¶

Is `self` a keyword?¶

How is `self` passed automatically?¶

Why does `User.greet()` raise an error?¶