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Understanding the Difference Between Shallow Copy and Deep Copy in Python

In Python, copying objects is a fundamental concept that often arises during programming, especially when handling mutable objects such as lists, dictionaries, or custom classes. Understanding how to effectively manage copies of these objects is crucial for writing efficient and bug-free code. The necessity of copying stems from the mutable nature of certain data structures, which means their content can be altered after they are created. Without proper copying techniques, modifications to these objects can lead to unintended side effects, as changes in one part of the code might inadvertently affect another part.

Python provides two primary mechanisms for copying objects: shallow copy and deep copy. Both methods serve the purpose of duplicating objects, but they operate differently, leading to distinct outcomes. A shallow copy creates a new object, but inserts references into it to the objects found in the original. This means that while the new object is a separate entity, the elements within it are still references to the same objects within the original. On the other hand, a deep copy creates a new object and recursively copies all objects found in the original. Consequently, the new object and all objects within it are entirely independent of the original.

These differences are pivotal when dealing with nested or complex structures in Python, as the choice between shallow and deep copy can significantly influence the behavior of your code. Grasping the nuances between these two types of copies allows developers to make informed decisions and avoid common pitfalls associated with object mutability. The following sections will delve deeper into the mechanics of shallow and deep copies, providing detailed explanations and examples to highlight their respective use-cases and implications in Python programming.

What is a Shallow Copy?

A shallow copy in Python refers to the process of creating a new object while inserting references to the objects found in the original object. Essentially, when a shallow copy is made, Python constructs a new collection object and populates it with references to the child objects found in the original. Therefore, the new object contains references to the same elements as the original one, rather than duplicating the elements themselves.

To illustrate, consider a list containing other lists as its elements. A shallow copy of this list will create a new outer list, but the inner lists will still be referenced from the original list. This means that changes made to any mutable objects within the shallow copy will propagate back to the original object. As a result, modifications to these nested objects will reflect in both the original and the shallow copy, demonstrating the reference-based nature of shallow copying.

The `copy` module in Python provides a method called `copy()` to create a shallow copy. For instance, using `copy.copy(original_list)` will achieve this. It is important to note that while the outer structure is duplicated, the inner objects are not. This behavior is particularly useful when the structure of the object is to be replicated without the need to duplicate the nested objects themselves, thus saving memory and processing time.

Shallow copies are beneficial in scenarios where the referenced objects are immutable, such as tuples or strings, since changes to these types will not affect the original. However, caution is necessary when dealing with mutable objects like lists or dictionaries. Understanding this distinction is crucial for developers working with Python, especially in complex applications where data integrity and memory management are paramount.

In summary, shallow copying is a technique that creates a new object, inserting references to the original’s elements. This results in partial duplication, where the outer structure is new, but the inner objects remain shared between the original and the copy. This behavior underscores the importance of recognizing the implications of shallow copying, particularly concerning mutable objects.

What is a Deep Copy?

A deep copy in Python refers to the process of creating a new object that is a complete, independent replica of the original object. Unlike a shallow copy, which only duplicates the object’s top-level structure, a deep copy goes further by recursively copying all nested objects found within the original. This ensures that the new, duplicated object is entirely distinct from the original, with no shared references.

When performing a deep copy, the copy.deepcopy() function from Python’s copy module is typically used. This function traverses through the original object and creates new instances of all objects it encounters, including any nested lists, dictionaries, or other complex data structures. As a result, modifications made to the deep-copied object do not impact the original object in any way, preserving the integrity of the original data.

Consider a scenario where we have a complex object, such as a list containing dictionaries, which in turn hold lists of their own. Utilizing a shallow copy would only duplicate the outermost list, leaving the nested dictionaries and lists as references to the original objects. Consequently, changes to these nested elements in the copied list would reflect in the original. On the other hand, a deep copy would create entirely new instances of the nested dictionaries and lists, ensuring complete independence from the original structure.

This behavior of deep copies can be particularly advantageous in scenarios where the data structure is complex and changes to the copy should not affect the original. For instance, when working with configurations or state information in large applications, using a deep copy can help prevent unintended side effects and maintain data consistency.

Whether you are coding in Python in Ranchi or any other location, understanding the distinction between shallow and deep copies is crucial for effective data manipulation and ensuring the reliability of your programs.

Creating a Shallow Copy in Python

In Python, a shallow copy of an object is a new object that is a copy of the original, but shares references to the objects contained within. Essentially, it creates a new instance of the object with the contents being references to the elements of the original object. This means that changes made to the mutable elements of the shallow copy will reflect in the original object and vice versa.

To create a shallow copy in Python, the `copy` module’s `copy()` method is commonly used. Here is an example:

import copy
original_list = [1, 2, [3, 4]]
shallow_copied_list = copy.copy(original_list)

In the above code snippet, `shallow_copied_list` is a shallow copy of `original_list`. If you modify the top-level elements of `shallow_copied_list`, the `original_list` remains unaffected. However, changes to nested elements will affect both the original and the shallow copy.

Consider this example to further illustrate:

shallow_copied_list[0] = 'a'
# Output: [1, 2, [3, 4]]

Now, let’s modify a nested element:

shallow_copied_list[2][0] = 'b'
# Output: [1, 2, ['b', 4]]

As seen, changing the nested element in `shallow_copied_list` also changed the nested element in `original_list`. This is because the shallow copy maintains references to the same objects at the nested level.

Scenarios where a shallow copy is sufficient include cases where the object contains immutable elements or when you only need to duplicate the top-level structure of the object. It is particularly useful for copying lists or dictionaries with simple, flat data structures, where nested elements do not need independent duplication.

Creating a Deep Copy in Python

When working with complex data structures in Python, such as nested lists or dictionaries, it is often necessary to create an independent copy that does not affect the original object. This is where deep copies come into play. A deep copy creates a new object and recursively adds copies of nested objects found in the original. Python provides an intuitive way to accomplish this using the `deepcopy()` method from the `copy` module.

To create a deep copy, you first need to import the `copy` module:

import copy

Next, you can create a deep copy of an object by using the `deepcopy()` method as shown in the following example:

original_list = [[1, 2, 3], [4, 5, 6]]deep_copied_list = copy.deepcopy(original_list)

In this case, `deep_copied_list` is a new list that contains copies of the nested lists from `original_list`. Modifying the `deep_copied_list` will not affect the `original_list`, and vice versa:

deep_copied_list[0][0] = 99print(original_list)# Output: [[1, 2, 3], [4, 5, 6]]print(deep_copied_list)# Output: [[99, 2, 3], [4, 5, 6]]

Using deep copies is particularly beneficial when dealing with mutable objects in Python. They ensure that changes made to the copied object do not inadvertently alter the original. This is essential in scenarios such as data processing pipelines, where maintaining the integrity of the original data is crucial.

However, it is important to consider the performance implications of using deep copies. Because the `deepcopy()` method recursively copies every nested object, it can be significantly slower and more memory-intensive than a shallow copy, especially for large and deeply nested structures. Therefore, deep copies should be used judiciously, particularly in performance-critical applications.

In conclusion, understanding when and how to use deep copies in Python can greatly enhance the robustness of your code. By leveraging the `deepcopy()` method from the `copy` module, you can ensure that your complex data structures are manipulated safely and independently, preserving the original data’s integrity.

Practical Examples and Use Cases

When working with Python, understanding how to effectively use shallow and deep copies can be crucial in various real-world scenarios. To illustrate, let us consider a scenario involving a list of lists, a common data structure in Python. Suppose we have a matrix represented as a list of lists, and we need to create a copy for modification purposes without altering the original matrix.

A shallow copy, created using the `copy()` method or the `copy` module’s `copy` function, copies the top-level structure of the list. However, it doesn’t create copies of nested objects. For example:

import copymatrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]shallow_copy_matrix = copy.copy(matrix)shallow_copy_matrix[0][0] = 100

In this case, modifying `shallow_copy_matrix[0][0]` also changes the value in the original `matrix`, because the nested lists are still references to the same objects.

In contrast, a deep copy, created using the `deepcopy` function from the `copy` module, duplicates all objects recursively, ensuring that nested objects are also copied:

deep_copy_matrix = copy.deepcopy(matrix)deep_copy_matrix[0][0] = 100

Here, modifying `deep_copy_matrix[0][0]` does not affect `matrix`, since `deep_copy_matrix` is an entirely independent copy.

These examples highlight the importance of understanding the distinction: shallow copies are suitable for simpler structures or when nested objects’ immutability can be assured, while deep copies are essential for complex, nested data structures where independent modifications are required.

A common pitfall is assuming that a shallow copy is sufficient when deep copying is necessary, leading to unintended side effects. Best practices dictate using shallow copies for performance when dealing with large data structures, provided the nested objects will not be modified. Otherwise, deep copies are more appropriate despite their higher memory and computational costs.

In practice, choosing between shallow and deep copies in Python is a balancing act between performance and the need for independent data structures. Understanding these concepts and their implications helps developers write more efficient and bug-free code in various applications.

Performance Considerations

When working with copies of objects in Python, it is crucial to understand the performance implications associated with shallow and deep copying. Each method has distinct memory and computational overheads that can significantly impact the efficiency of your code, especially when dealing with large datasets or complex structures.

Shallow copies are generally faster and consume less memory compared to deep copies. This is because a shallow copy only creates a new object and inserts references to the objects found in the original. As a result, the creation of a shallow copy involves less computational effort and memory allocation. However, this speed and efficiency come at the cost of potential unintended side effects, as changes to mutable objects within the shallow copy will reflect in the original object.

On the other hand, deep copies are more resource-intensive. A deep copy creates a new object and recursively copies all objects found in the original, resulting in significant memory and computational overhead. This approach ensures complete independence from the original object, which is beneficial when you need to modify the copied objects without affecting the original ones. The trade-off, however, is the increased time and memory consumption required to duplicate the entire structure.

To optimize performance when copying objects in Python, it is essential to consider the specific requirements of your application. If the objects you are working with are relatively small or require frequent copying, shallow copies may be more appropriate. Conversely, for large and complex objects where independence from the original is critical, deep copies might be the better choice.

Additionally, consider alternative strategies such as using immutable data structures where possible, minimizing the need for copies, or leveraging Python’s built-in methods and libraries that offer optimized copying mechanisms. By carefully evaluating the performance implications and choosing the appropriate copying method, you can enhance both the efficiency and reliability of your Python applications.


In this blog post, we have explored the fundamental differences between shallow copy and deep copy in Python. Understanding these concepts is crucial for any developer aiming to manipulate data structures efficiently. We first discussed that a shallow copy creates a new object but inserts references into it that point to the same objects found in the original collection. This means that changes to mutable objects within the copied collection will reflect in the original.

On the other hand, a deep copy generates a new object and recursively copies all objects found in the original, ensuring that modifications to the new collection do not affect the original one. This distinction is vital when working with nested data structures or when you need to maintain data integrity across different parts of a Python application.

Recognizing when to use shallow copy versus deep copy can optimize memory usage and improve the performance of your Python programs, particularly in complex projects. Experimenting with these copying techniques in your own Python projects can help solidify your understanding and enable you to make more informed decisions about data manipulation.

By mastering the nuances of shallow and deep copies, developers can write more robust, efficient, and maintainable code, which is especially beneficial for those working in tech hubs like Ranchi. As you continue to grow your Python skill set, remember that knowing when to apply these concepts can significantly impact the quality and efficiency of your code.

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