How to Deep Copy a Dictionary in Python: Syntax and Examples

To deep copy a dictionary in Python, use the deepcopy() function from the copy module. This creates a new dictionary with all nested objects copied, so changes to the copy do not affect the original.

Syntax

Use the deepcopy() function from the copy module to create a deep copy of a dictionary. This copies the dictionary and all nested objects inside it.

• import copy: imports the copy module
• copy.deepcopy(original_dict): returns a deep copy of original_dict

python

import copy
new_dict = copy.deepcopy(original_dict)

Example

This example shows how modifying a nested list inside a deep copied dictionary does not affect the original dictionary.

python

import copy

original = {'numbers': [1, 2, 3], 'letters': ['a', 'b', 'c']}
copy_dict = copy.deepcopy(original)

copy_dict['numbers'].append(4)

print('Original:', original)
print('Copy:', copy_dict)

Output

Original: {'numbers': [1, 2, 3], 'letters': ['a', 'b', 'c']}
Copy: {'numbers': [1, 2, 3, 4], 'letters': ['a', 'b', 'c']}

Common Pitfalls

A common mistake is using dict.copy() or copy.copy() which only create shallow copies. Shallow copies duplicate the top-level dictionary but keep references to nested objects, so changes inside nested objects affect both dictionaries.

Always use copy.deepcopy() when you want a fully independent copy including nested objects.

python

import copy

original = {'data': [1, 2, 3]}
shallow_copy = original.copy()  # or copy.copy(original)
shallow_copy['data'].append(4)

print('Original after shallow copy change:', original)

# Correct way

deep_copy = copy.deepcopy(original)
deep_copy['data'].append(5)
print('Original after deep copy change:', original)

Output

Original after shallow copy change: {'data': [1, 2, 3, 4]}
Original after deep copy change: {'data': [1, 2, 3, 4]}

Quick Reference

• Shallow copy: dict.copy() or copy.copy() - copies only top-level dictionary
• Deep copy: copy.deepcopy() - copies dictionary and all nested objects
• Use deep copy to avoid shared references in nested data

Key Takeaways

Use copy.deepcopy() to create a fully independent copy of a dictionary including nested objects.

Shallow copies only duplicate the top-level dictionary and keep references to nested objects.

Modifying nested objects in a shallow copy affects the original dictionary.

Always import the copy module before using deepcopy.

Deep copying is essential when working with complex nested data structures.