Shallow Copy vs Deep Copy in Python: The copy Module0%

Shallow Copy vs Deep Copy in Python: The copy Module

Intermediate9 min read•Updated: 2026-10-23

🐍 Object Mutability: Shallow Copy vs Deep Copy

In Python, variables do not store actual values—they store references (memory addresses) to objects. Failing to understand the difference between assignment, shallow copying, and deep copying is one of the most common causes of subtle data corruption bugs in intermediate Python programs.

🧩 1. Assignment (`=`) vs Copying

When you assign a variable using =, you do not copy the object. You merely create a new name pointing to the exact same object in memory:

1original = [10, 20, 30]
2alias = original
3 
4alias.append(99)
5 
6print("Original:", original) # [10, 20, 30, 99]
7print("Alias:   ", alias)    # [10, 20, 30, 99]
8print(original is alias)     # True (identical memory address!)

📄 2. Shallow Copy (`copy.copy` or `.copy()`)

A Shallow Copy creates a new outer collection, but inserts references to the same child objects found in the original:

1import copy
2 
3original = [[1, 2, 3], [4, 5, 6]]
4shallow = copy.copy(original)
5 
6# Modifying the outer container does NOT affect original:
7shallow.append([7, 8, 9])
8print(len(original)) # 2 (unaffected outer container)
9 
10# BUT modifying an inner nested list DOES affect both!
11shallow[0].append(999)
12print(original[0])   # [1, 2, 3, 999]  <-- ACCIDENTAL MUTATION!

Because the nested child lists were only referenced, both original[0] and shallow[0] point to the identical inner list in memory.

🧬 3. Deep Copy (`copy.deepcopy`)

A Deep Copy constructs a brand-new compound object and then recursively copies all nested child objects found within it. Modifications to any level of the clone have zero impact on the original:

1import copy
2 
3original = [[1, 2, 3], [4, 5, 6]]
4deep = copy.deepcopy(original)
5 
6deep[0].append(999)
7 
8print("Original[0]:", original[0]) # [1, 2, 3]       (Protected!)
9print("Deep[0]:    ", deep[0])     # [1, 2, 3, 999]   (Isolated clone)

⚖️ Summary Comparison

Method	Syntax	Outer Object	Nested Child Objects
Assignment	`b = a`	Shared reference	Shared reference
Shallow Copy	`b = a.copy()` or `copy.copy(a)`	Independent clone	Shared references
Deep Copy	`b = copy.deepcopy(a)`	Independent clone	Independent recursive clones

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

In Python, what does the expression b = a do when a is a list?

Question 2

Which module from Python's standard library provides shallow and deep copy utilities?

Question 3

What is the primary risk of using a shallow copy on a nested list?

Question 4

Which function creates a completely independent clone of an object along with all its nested objects?

Question 5

What does the is operator verify in x is y?

🚀 What's Next?

In the next lesson, we will implement our full OOP understanding in Project: Bank Account Class System (5: Object-Oriented Programming (Intermediate)).

👉 Continue to Next Lesson: Project: Bank Account Class System →

Next Lesson

Project: Bank Account Class System

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Dataclasses: Clean Data Models with @dataclass	Project: Bank Account Class System

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

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Project: Bank Account Class System

Shallow Copy vs Deep Copy in Python: The copy Module

Intermediate9 min read•Updated: 2026-10-23

Study Materials Cheatsheet

🐍 Object Mutability: Shallow Copy vs Deep Copy

🧩 1. Assignment (`=`) vs Copying

When you assign a variable using =, you do not copy the object. You merely create a new name pointing to the exact same object in memory:

1original = [10, 20, 30]
2alias = original
3 
4alias.append(99)
5 
6print("Original:", original) # [10, 20, 30, 99]
7print("Alias:   ", alias)    # [10, 20, 30, 99]
8print(original is alias)     # True (identical memory address!)

📄 2. Shallow Copy (`copy.copy` or `.copy()`)

A Shallow Copy creates a new outer collection, but inserts references to the same child objects found in the original:

1import copy
2 
3original = [[1, 2, 3], [4, 5, 6]]
4shallow = copy.copy(original)
5 
6# Modifying the outer container does NOT affect original:
7shallow.append([7, 8, 9])
8print(len(original)) # 2 (unaffected outer container)
9 
10# BUT modifying an inner nested list DOES affect both!
11shallow[0].append(999)
12print(original[0])   # [1, 2, 3, 999]  <-- ACCIDENTAL MUTATION!

Because the nested child lists were only referenced, both original[0] and shallow[0] point to the identical inner list in memory.

🧬 3. Deep Copy (`copy.deepcopy`)

1import copy
2 
3original = [[1, 2, 3], [4, 5, 6]]
4deep = copy.deepcopy(original)
5 
6deep[0].append(999)
7 
8print("Original[0]:", original[0]) # [1, 2, 3]       (Protected!)
9print("Deep[0]:    ", deep[0])     # [1, 2, 3, 999]   (Isolated clone)

⚖️ Summary Comparison

Method	Syntax	Outer Object	Nested Child Objects
Assignment	`b = a`	Shared reference	Shared reference
Shallow Copy	`b = a.copy()` or `copy.copy(a)`	Independent clone	Shared references
Deep Copy	`b = copy.deepcopy(a)`	Independent clone	Independent recursive clones

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

In Python, what does the expression b = a do when a is a list?

Question 2

Which module from Python's standard library provides shallow and deep copy utilities?

Question 3

What is the primary risk of using a shallow copy on a nested list?

Question 4

Which function creates a completely independent clone of an object along with all its nested objects?

Question 5

What does the is operator verify in x is y?

🚀 What's Next?

In the next lesson, we will implement our full OOP understanding in Project: Bank Account Class System (5: Object-Oriented Programming (Intermediate)).

👉 Continue to Next Lesson: Project: Bank Account Class System →

Next Lesson

Project: Bank Account Class System

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Dataclasses: Clean Data Models with @dataclass	Project: Bank Account Class System

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

🐍 Object Mutability: Shallow Copy vs Deep Copy

🧩 1. Assignment (=) vs Copying

📄 2. Shallow Copy (copy.copy or .copy())

🧬 3. Deep Copy (copy.deepcopy)

⚖️ Summary Comparison

Practice Quiz

🚀 What's Next?

Project: Bank Account Class System

Related Lessons

Practice Quiz

Loading Page Content...

🐍 Object Mutability: Shallow Copy vs Deep Copy

🧩 1. Assignment (=) vs Copying

📄 2. Shallow Copy (copy.copy or .copy())

🧬 3. Deep Copy (copy.deepcopy)

⚖️ Summary Comparison

Practice Quiz

🚀 What's Next?

Project: Bank Account Class System

Related Lessons

Practice Quiz

🧩 1. Assignment (`=`) vs Copying

📄 2. Shallow Copy (`copy.copy` or `.copy()`)

🧬 3. Deep Copy (`copy.deepcopy`)

🧩 1. Assignment (`=`) vs Copying

📄 2. Shallow Copy (`copy.copy` or `.copy()`)

🧬 3. Deep Copy (`copy.deepcopy`)