Mutable vs Immutable Objects in Python (Explained Simply)

Mutable vs Immutable Objects in Python

I’ve added a simple visual comparing mutable and immutable objects.

Keep it in mind while reading — it helps anchor the mental model.

This post explains a core Python concept that often causes confusion, especially when working with data structures, recursion, and backtracking.

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1️⃣ Types of Python Objects

In Python, objects can be broadly classified into two categories based on whether their internal state can change:

• Mutable objects
• Immutable objects

Understanding this distinction is critical for writing correct and predictable Python code.

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2️⃣ Mutable Objects

Mutable objects are objects whose contents can be changed after creation.

When you modify a mutable object:

• The same object in memory is updated
• The memory address does not change

Common Mutable Objects

• list
• set
• dict

Example

a = [1, 2, 3]
a.append(4)
print(a)  # [1, 2, 3, 4]

Here:

• a still points to the same list
• The list’s contents are modified in place

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3️⃣ Immutable Objects

Immutable objects are objects whose contents cannot be changed after creation.

Any operation that appears to modify them actually:

• Creates a new object
• Assigns a new memory address

Common Immutable Objects

• int
• float
• str
• tuple
• frozenset

Example

x = 10
x = x + 1
print(x)  # 11

What happened:

• 10 was not modified
• A new integer 11 was created
• x now points to a new object

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4️⃣ Why This Difference Matters

This distinction affects:

• Function arguments
• Recursion and backtracking
• Shared references
• Unexpected side effects

Example: Function Behavior

def add_item(lst):
    lst.append(100)

nums = [1, 2, 3]
add_item(nums)
print(nums)  # [1, 2, 3, 100]

The list changed outside the function because it is mutable.

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5️⃣ Mental Model (Remember This)

Mutable objects change in place

Immutable objects create new values

If you remember this, most bugs around this topic disappear.

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TL;DR

• Mutable objects can be changed after creation
• Immutable objects cannot
• Lists, sets, dicts → mutable
• Ints, strings, tuples → immutable
• Mutability affects function behavior and bugs

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Understanding mutability early will save you hours of debugging later.