How to Copy Dictionaries in Python

When working with dictionaries in Python, there are situations where you might want to create a copy of a dictionary. Copying dictionaries allows you to modify the copied version without affecting the original one. Python provides several ways to achieve this, depending on whether you need a shallow copy or a deep copy.

In this article, we’ll explore different methods to copy dictionaries, including the nuances between shallow and deep copies, to help you make the right choice for your specific needs.

Table of Contents:

1. Understanding Shallow vs. Deep Copy
2. Copying a Dictionary with copy()
3. Using the dict() Constructor
4. Copying Nested Dictionaries with deepcopy()
5. Best Practices for Copying Dictionaries

1. Understanding Shallow vs. Deep Copy

Before diving into the methods for copying dictionaries, it’s important to understand the difference between a shallow copy and a deep copy.

• Shallow Copy: A shallow copy of a dictionary creates a new dictionary object, but it only copies references to the original data. If the dictionary contains mutable objects (e.g., lists, other dictionaries), changes to these objects will affect both the original and the copied dictionary.
• Deep Copy: A deep copy, on the other hand, creates a new dictionary object and recursively copies all objects inside the original dictionary. This ensures that changes to nested objects in the copied dictionary do not affect the original one.

2. Copying a Dictionary with copy()

The copy() method creates a shallow copy of the dictionary. This is the most common way to copy a dictionary when you don’t need to worry about modifying nested data structures.

Example:

Let’s create a dictionary representing a book's details.

book = {
    'title': 'Python 101',
    'author': 'John Smith',
    'year': 2020
}

To make a shallow copy of this dictionary, use the copy() method:

book_copy = book.copy()

Now, book_copy is an independent dictionary with the same content as book. Changes to book_copy will not affect book:

book_copy['year'] = 2021
print(book_copy)  # Output: {'title': 'Python 101', 'author': 'John Smith', 'year': 2021}
print(book)       # Output: {'title': 'Python 101', 'author': 'John Smith', 'year': 2020}

This method is quick and efficient for flat dictionaries that do not contain nested structures.

3. Using the dict() Constructor

Another way to create a shallow copy of a dictionary is to use the dict() constructor. This method is functionally equivalent to using the copy() method.

Example:

book_copy = dict(book)

This achieves the same result as copy(), creating a new dictionary with the same key-value pairs as book. The dict() constructor is a clean and readable alternative that some developers prefer.

4. Copying Nested Dictionaries with deepcopy()

If your dictionary contains nested dictionaries or other mutable objects like lists, and you want to ensure that changes to the copied dictionary do not affect the original, you’ll need to perform a deep copy. This can be done using the deepcopy() method from Python’s copy module.

Example:

Let’s create a nested dictionary representing a library with book details.

import copy

library = {
    'section_1': {
        'title': 'Python 101',
        'author': 'John Smith'
    },
    'section_2': {
        'title': 'Data Science Essentials',
        'author': 'Jane Doe'
    }
}

To make a deep copy of the library dictionary, use deepcopy():

library_copy = copy.deepcopy(library)

Now, even if you modify nested data in library_copy, the original library will remain unaffected:

library_copy['section_1']['title'] = 'Advanced Python'
print(library_copy['section_1']['title'])  # Output: 'Advanced Python'
print(library['section_1']['title'])       # Output: 'Python 101'

This method ensures that all nested objects are also copied, giving you complete independence between the original and the copied dictionary.

5. Best Practices for Copying Dictionaries

• Choose the Right Method: Use copy() or the dict() constructor for shallow copies when your dictionary does not contain nested mutable objects. If your dictionary is nested or complex, use deepcopy() to ensure you don’t inadvertently change the original data.
• Be Mindful of Performance: Shallow copies are faster and consume less memory compared to deep copies. Use deep copying only when necessary, as it requires more resources to duplicate all nested structures.
• Avoid Manual Copying: Manually copying dictionaries by iterating through keys is error-prone and inefficient. Rely on Python’s built-in methods like copy() and deepcopy() for a more reliable and cleaner solution.

Conclusion

Copying dictionaries in Python is a straightforward task, but choosing the right method depends on the complexity of your data. For flat dictionaries, copy() and dict() provide a quick and efficient way to create shallow copies. When dealing with nested dictionaries, deepcopy() ensures that you have complete separation between the original and copied data.