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

Modifying List While Iterating

When working with lists in Python, it's imperative to understand the consequences of modifying them while looping. Consider the following code:

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

Unexpectedly, this code produces an incorrect output. Why does this occur?

When looping through a list in Python, the iterator remembers the current position and uses it to iterate over the elements. However, if the list is modified during iteration, the iterator's memory is not updated. As a result, the loop may skip or duplicate elements.

In the above example, l.pop(0) removes the first element from the list. The loop therefore continues to the second element, but the iterator still remembers the first element. This leads to the loop printing the first element twice.

To avoid this issue, it's crucial to never modify the list while iterating. Instead, you can use a temporary variable or a copy of the list for modifications.

Alternatively, you can consider using a while loop with an index variable, as in this example:

i = 0
while i < len(l):
    print(l[i])
    i += 1

This method ensures that the loop correctly iterates through the list, even if elements are removed during the process.

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