The Secret Life of Python: The Loophole

Source: Dev.to

The trap of modifying a list while iterating over it (and how to fix it)

Timothy was staring at a list of sensor data on his screen.

“I told it to remove the errors. I wrote the code explicitly. But it keeps missing them.”

He had a list of temperature readings where any value below zero was considered an error. His loop looked like this:

# Timothy's "Cleanup" Loop
temperatures = [10, -5, -2, 15, 20]

for temp in temperatures:
    if temp = 0]

print(f"Cleaned List: {clean_temperatures}")

Output

Cleaned List: [10, 15, 20]

This approach avoids in‑place modification altogether and is often faster and clearer.

Margaret’s Cheat Sheet

• The Trap: Never add or remove items from a list while looping over it.
• The Symptom: Skipped items (when removing) or infinite loops (when adding).
• The Reason: Changing the list length shifts indices, but the loop counter continues marching forward.

Fixes

• Iterate over a copy: for item in my_list[:] (quick fix).
• List comprehension: [x for x in my_list if condition] (best for filtering).
• Iterate backwards: for item in reversed(my_list): (advanced; useful when you must modify in‑place without copying).