Mutable vs Immutable Objects — Overview

What you'll learn

Topic	Focus
Mutable objects	Edit data in place
Immutable objects	Create a new object
Item assignment	Why some types raise TypeError
id()	Identify an object's memory identity

After String Handling, you already saw that strings are read-only. This lesson generalizes that idea to all Python objects.

Visual roadmap

The easiest way to remember the difference:

Memory rule: Mutable = modify · Immutable = replace

1. What is a Mutable Object?

A mutable object allows its content to change without creating a new object.

lst = [10, 20, 30]

print(id(lst))

lst[0] = 99

print(lst)
print(id(lst))

Expected pattern (your exact numbers will differ):

4362815488        # example id

[99, 20, 30]

4362815488        # same id — list was modified in place

Notice id() stays the same because the list itself was modified.

2. What is an Immutable Object?

An immutable object cannot change its content. Any modification creates a new object.

s = "python"

print(id(s))

s = s.upper()

print(s)
print(id(s))

The variable name s is reused, but it now points to a different object.

3. Item Assignment Rule

A quick way to identify mutability is to ask: can I do obj[index] = value?

Mutable (allowed)

lst = [10, 20, 30]

lst[0] = 99      # Works

Immutable (not allowed)

s = "python"

# s[0] = "P"     # TypeError

Rule: Mutable types support item assignment. Immutable types do not.

4. Mutable vs Immutable Comparison

Feature	Mutable	Immutable
Can modify content?	Yes	No
Memory address changes?	Usually no	Yes (new object)
Item assignment	Allowed	TypeError
Example	list	str

Visual memory:

5. Full Python Type Classification

Mutable types

These objects can change after creation.

Type	Role
list	Ordered collection
dict	Key-value mapping
set	Unique unordered values
bytearray	Editable bytes

Immutable types

These cannot change after creation.

Type	Role
int, float, bool, complex	Numeric values
str, bytes	Text and raw bytes
tuple, range	Fixed sequences
frozenset, NoneType	Fixed set / sentinel

Common interview question: set is mutable, but frozenset is immutable.

Deep dives: Immutable types · Mutable types

6. id() Demonstration

The id() function returns the identity (memory reference) of an object.

s = "python"

print("Before:", id(s))

s = s.upper()

print("After :", id(s))

Conceptual pattern:

Before: 10123456
After : 10198765    # different — new object

Different id() values mean Python created a new object. Related: is / is not.

Playground

Run this complete example to compare both behaviors.

s = "python"

print("str id:", id(s))

s = s.upper()

print("after upper id:", id(s))

lst = [10, 20, 30]

print("list id before:", id(lst))

lst[0] = 99

print("list id after:", id(lst))

Observe:

• str → ID changes
• list → ID stays the same

Use the lesson playground at the bottom of the page to run this code live.

Quick knowledge check

1. Can you run s[0] = "P" on a string?
   Answer: No — strings are immutable and raise TypeError.

2. Does lst[0] = 99 create a new list?
   Answer: No — it modifies the existing list.

3. Which function shows an object's identity?
   Answer: id(obj)

30-second interview revision

Remember	Meaning
Mutable	Content changes at the same id()
Immutable	New object is created
list	Mutable
str	Immutable
id(obj)	Shows object identity

Interview shortcut: If obj[i] = value works, the object is usually mutable. If it raises TypeError, it is immutable.

Next: Immutable Types in Python.