Day 16:Modifying a list while looping over it

 

🐍Python Mistakes Everyone Makes ❌

Day 16: Modifying a List While Looping Over It

One common Python pitfall is changing a list while iterating over it. This often leads to skipped elements and unexpected results.

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❌ The Mistake

numbers = [1, 2, 3, 4]

for n in numbers:
      if n % 2 == 0:
           numbers.remove(n)

print(numbers)

This code does not behave as expected.

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✅ The Correct Way

numbers = [1, 2, 3, 4]

for n in numbers[:]: # loop over a copy
      if n % 2 == 0:
           numbers.remove(n)

print(numbers)

By looping over a copy of the list, the original list can be safely modified.

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❌ Why This Fails?

When you modify a list while looping over it, Python’s iterator gets out of sync.
This causes elements to be skipped or processed incorrectly.

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✔ Key Points

• Modifying a list during iteration causes logic bugs

• Iteration order changes when elements are removed

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🧠 Simple Rule to Remember

• Don’t modify a list while looping over it

• Loop over a copy or create a new list

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🔑 Key Takeaway

If you need to filter or modify a list, prefer:

• looping over a copy (numbers[:])

• or using list comprehensions for cleaner, safer code