How to Copy and Modify Lists in Python Without Linking Them

Learn how to copy a list multiple times in Python and make modifications without affecting the original list or others.
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This video is based on the question https://stackoverflow.com/q/69621075/ asked by the user 'Natan' ( https://stackoverflow.com/u/17184513/ ) and on the answer https://stackoverflow.com/a/69621120/ provided by the user 'ddejohn' ( https://stackoverflow.com/u/6298712/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Copy and Modify Lists in Python Without Linking Them

When working with lists in Python, especially when you're dealing with complex data structures like nested lists, one common problem you might face is how to effectively duplicate a list, modify one of them, and ensure the others remain unchanged. This is particularly relevant when dealing with collections of data that require individual manipulation without unintentional side effects. In this guide, we will explore this issue and provide a clear solution.

The Problem: Unexpected Linking of Lists

Imagine you have a list named cubes, which is a nested structure, meaning it contains other lists as its elements. You want to create multiple copies of this list but encounter a surprising issue: modifying one of the copies seems to affect all the others. This happens because you're not creating true independent copies of your list. Here's a quick look at the code that illustrates this problem:

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When you run this code, all six lists in sides will reference the same objects defined in cubes. This behavior is typical of shallow copies. To better understand this, let's delve into the solution.

The Solution: Using deepcopy

To solve the issue of linked copies, we need to create deep copies of our nested list structure. The deepcopy function from Python's built-in copy module serves this purpose effectively. Here's how to implement it:

Step-by-Step Solution

Import the deepcopy Function:
First, you'll need to import the deepcopy function from the copy module.

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Create Independent Copies:
Instead of using the .copy() method, use deepcopy() in a list comprehension to generate your independent copies:

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Revised Code

Here’s how the modified function looks with deepcopy in place:

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Conclusion

By using deepcopy(), you can easily create separate copies of a list to manipulate each one independently without risking unintentional changes to other copies. This technique is essential when working with nested structures in Python. Next time you face the challenge of copying lists, remember: shallow copies are not always the best solution—opt for deepcopy to maintain independence amongst your data structures!