Day 17:Assuming list copy = deep copy

 

🐍 Python Mistakes Everyone Makes ❌

🐍 Day 17: Assuming list.copy() = Deep Copy

Copying lists in Python can be tricky especially when nested lists are involved.

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❌ The Mistake

a = [[1, 2], [3, 4]]
b = a.copy()

b[0].append(99)
print(a)

You might expect a to remain unchanged, but it doesn’t.

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❌ Why This Fails?

list.copy() creates a shallow copy.
That means:

• The outer list is copied

• Inner (nested) lists are still shared

So modifying a nested list in b also affects a.

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✅ The Correct Way

import copy

a = [[1, 2], [3, 4]]
b = copy.deepcopy(a) 

b[0].append(99)
print(a)

Now a stays untouched because everything is fully copied.

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✔ Key Takeaways

✔ list.copy() → shallow copy
✔ Nested objects remain shared
✔ Use deepcopy() for independent copies

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🧠 Simple Rule to Remember

🐍 Shallow copy → shared inner objects
🐍 Deep copy → fully independent copy