Shallow Copy vs Deep Copy in Python: Key Differences and Usage

In Python, a shallow copy creates a new object but inserts references to the original objects inside it, while a deep copy creates a new object and recursively copies all objects found inside. This means changes to nested objects affect shallow copies but not deep copies.

Quick Comparison

Here is a quick side-by-side comparison of shallow copy and deep copy in Python.

Factor	Shallow Copy	Deep Copy
Copy Type	Copies top-level object only	Copies object and all nested objects recursively
Nested Objects	References to original nested objects	New copies of nested objects
Module/Function	copy.copy()	copy.deepcopy()
Effect of Changes in Nested Objects	Changes reflect in copy	Changes do NOT reflect in copy
Performance	Faster, less memory	Slower, more memory
Use Case	When nested objects are immutable or shared	When full independent copy needed

Key Differences

A shallow copy duplicates only the outer object, but the inner objects remain shared between the original and the copy. This means if you modify a nested object inside the copy, the change will also appear in the original object because both point to the same nested object.

In contrast, a deep copy duplicates the outer object and recursively copies all nested objects inside it. This creates a fully independent clone, so changes to any part of the copy do not affect the original object at all.

Python provides these operations through the copy module: copy.copy() for shallow copy and copy.deepcopy() for deep copy. Choosing between them depends on whether you want to share nested objects or have completely separate copies.

Shallow Copy Code Example

python

import copy

original = [[1, 2], [3, 4]]
shallow = copy.copy(original)

shallow[0][0] = 100

print("Original after shallow copy change:", original)
print("Shallow copy:", shallow)

Output

Original after shallow copy change: [[100, 2], [3, 4]]
Shallow copy: [[100, 2], [3, 4]]

Deep Copy Equivalent

python

import copy

original = [[1, 2], [3, 4]]
deep = copy.deepcopy(original)

deep[0][0] = 100

print("Original after deep copy change:", original)
print("Deep copy:", deep)

Output

Original after deep copy change: [[1, 2], [3, 4]]
Deep copy: [[100, 2], [3, 4]]

When to Use Which

Choose shallow copy when you want a new container object but are okay with sharing the nested objects inside it, especially if those nested objects are immutable or you want changes to reflect across copies.

Choose deep copy when you need a completely independent copy of an object and all objects it contains, so that changes in the copy do not affect the original at any level.

In summary, use shallow copy for performance and memory efficiency when full independence is not required, and deep copy when you need full separation.

Key Takeaways

Shallow copy duplicates only the outer object, sharing nested objects.

Deep copy duplicates the outer object and all nested objects recursively.

Use copy.copy() for shallow copy and copy.deepcopy() for deep copy.

Changes in nested objects affect shallow copies but not deep copies.

Choose deep copy when you need full independence between copies.