Why Does Modifying a List During Iteration in Python Lead to Unexpected Behavior?

Impact of Modifying Lists During Iteration

When working with lists in Python, it's crucial to be cautious about modifying them during iteration. Attempting to manipulate the original list while traversing it can lead to unpredictable outcomes.

Consider the following code sample:

l = range(100)
for i in l:
    print(i)
    print(l.pop(0))
    print(l.pop(0))

You might expect this code to print every element in the list and then remove the first two elements, repeating this process for the remaining elements. However, the actual output will deviate from this expectation.

The reason for this anomaly lies in the modification of the list during iteration. Removing elements from the list using l.pop(0) changes its size and order. As a result, the iterator i becomes outdated, causing the loop to behave erratically.

Avoiding List Modification During Iteration

To prevent such issues, it's advisable to create a copy of the list and iterate over the copy. This ensures that the original list remains unmodified, and the iterator provides accurate access to elements throughout the loop.

For instance, the following code demonstrates this approach:

l_copy = list(l)  # Create a copy of l
for i in l_copy:
    print(i)
    print(l.pop(0))
    print(l.pop(0))

Now, the output aligns with the expected behavior, as the original list l retains its integrity, while the modified copy l_copy is used for iteration.

Alternatively, as suggested in the answer provided, you can use a while loop with an index to control the iteration and make necessary modifications to the original list within the loop.

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