What is the difference between a shallow copy and a deep copy in Python?

Table of Contents

• Introduction
• Difference Between Shallow Copy and Deep Copy
  • 1. Shallow Copy
  • 2. Deep Copy
• Practical Examples
• Conclusion

Introduction

When working with complex objects in Python, such as lists or dictionaries, copying these objects can be tricky due to the way Python handles references. Understanding the difference between a shallow copy and a deep copy is essential for managing object references, especially when dealing with mutable objects like lists, sets, or dictionaries.

Difference Between Shallow Copy and Deep Copy

1. Shallow Copy

A shallow copy creates a new object but copies only the references to the elements within the original object. This means that for mutable objects (e.g., lists, dictionaries), changes made to the inner elements will reflect in both the original and the copied object since they both refer to the same underlying data.

• How it works: A new outer object is created, but its elements are still references to the original objects.

As seen in the example above, modifying an inner list in the shallow copy also changes the original list because both objects share references to the inner lists.

2. Deep Copy

A deep copy, on the other hand, creates a completely independent copy of both the outer object and all of the objects nested within it. This means that changes to the inner elements of the copied object do not affect the original object, and vice versa.

• How it works: A new object is created, and it recursively copies all objects contained within it.

In this case, modifying the deep copy does not affect the original list, as each element, including nested lists, has been copied independently.

Practical Examples

1. Shallow Copy: Use when you want to copy an object but still maintain shared references to the inner mutable objects. This is useful in cases where you want to save memory or intentionally share data.
2. Deep Copy: Use when you want to create a fully independent clone of the object and all of its contents. This is critical when working with nested structures, and you want to avoid unintended side effects caused by shared references.

Conclusion

In Python, a shallow copy only duplicates the outer object and keeps references to the original nested objects, whereas a deep copy recursively duplicates all objects. Understanding when to use shallow or deep copies is vital for managing mutable objects and ensuring that changes to copied data do not unintentionally affect the original data structure.