What is the difference between a deep copy and a shallow copy of a nested dictionary with nested mutable values in Python?
Antek N

In Python, when dealing with nested dictionaries that contain nested mutable values (such as lists or other mutable objects), understanding the difference between a deep copy and a shallow copy is important. Both copying methods create a new dictionary, but they differ in how they handle the nested objects. Here's a long-form explanation of the distinctions: Shallow Copy: 1. Purpose: - A shallow copy creates a new dictionary object but retains references to the nested objects within the original dictionary. - It allows multiple dictionaries to share the same nested objects, resulting in a shallow relationship. 2. Copying Behavior: - When a shallow copy is performed on a nested dictionary, a new dictionary object is created, and the top-level keys and values of the original dictionary are copied to the new dictionary. - However, the nested objects themselves are not duplicated. Instead, references to those objects are stored in the new dictionary. - This means that modifications made to the nested objects in the shallow copy will affect the original dictionary and vice versa. 3. Creating a Shallow Copy: - To create a shallow copy, you can use thecopy() method or thedict() constructor with the original dictionary as an argument. - Example:

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import copy

original_dict = {'key': [1, 2, 3]}

# Using copy()
shallow_copy = original_dict.copy()

# Using dict() constructor
shallow_copy = dict(original_dict)

Deep Copy: 1. Purpose: - A deep copy creates a completely independent copy of the original dictionary, including all nested objects. - It creates a new dictionary with its own copies of the nested objects, resulting in a deep and independent relationship. 2. Copying Behavior: - When a deep copy is performed on a nested dictionary, a new dictionary object is created, and new copies of the nested objects are recursively created and stored in the new dictionary. - This ensures that modifications made to the nested objects in the deep copy do not affect the original dictionary and vice versa. 3. Creating a Deep Copy: - To create a deep copy, you can use thedeepcopy() function from thecopy module. - Example:

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import copy

original_dict = {'key': [1, 2, 3]}

deep_copy = copy.deepcopy(original_dict)

To summarize: - A shallow copy creates a new dictionary but retains references to the nested objects, resulting in a shared relationship between the original dictionary and the copy. - A deep copy creates an independent copy of the original dictionary and all its nested objects, resulting in a deep and independent relationship. It's important to choose the appropriate copying method based on your specific requirements and the level of independence you need between the original dictionary and the copied dictionary.

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