You're Copying Lists Wrong in Python — Here's Why It Matters

🧠 What’s the Difference Between Shallow Copy and Deep Copy in Python?

If you’ve ever duplicated a list, dictionary, or any nested structure in Python and noticed unexpected changes in both the original and the copy, you’ve probably run into one of the most common (and confusing) behaviors in the language: the difference between shallow copy and deep copy.

In this article, we’ll break it down in a clear and practical way — with simple examples and real-world use cases.

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📋 What is a shallow copy?

A shallow copy only duplicates the outer object, while keeping references to the same inner objects. Think of it as copying the “shell” — but the internal contents still point to the originals.

Example:

import copy

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

shallow[0][0] = 99
print(original_list)
# Output: [[99, 2], [3, 4]]

✅ The outer list is copied
❌ The inner lists are still the same references as in the original

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🧬 What is a deep copy?

A deep copy performs a recursive copy of all objects — including any nested elements. This means you get an entirely independent structure.

Example:

import copy

original_list = [[1, 2], [3, 4]]
deep = copy.deepcopy(original_list)

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

✅ All contents are copied recursively
✅ Changes in one structure don’t affect the other

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🤔 When to use each one?

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⚠️ Common pitfalls

• Using copy.copy() on nested structures can cause silent bugs.

• Using list() or slicing ([:]) also creates a shallow copy, not deep.

• deepcopy is powerful but can be slower for large or complex objects.

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🔚 Conclusion

Knowing the difference between shallow and deep copy is essential to avoid subtle bugs — especially when working with complex data structures.

Whenever you copy an object, ask yourself:
➡️ “Do I want to copy just the outer structure, or everything inside too?”

Python gives you the power — and the responsibility — to make the right choice.

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Bruno Marques