Why Does Removing List Elements During Iteration in Python Lead to Unexpected Results?

Unexpected Behavior: Removing Elements from a List During Iteration

When attempting to iterate over a list and remove specific elements that meet certain criteria, an unexpected behavior may occur if the list is modified during the iteration process. Consider the following Python code:

numbers = list(range(1, 50))

for i in numbers:
    if i < 20:
        numbers.remove(i)

print(numbers)

Surprisingly, the result obtained is:

[2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]

We would expect the numbers below 20 to be removed, but they remain in the result. This is because the list is modified while it is being iterated over. When the first element, 1, is removed, the position of the subsequent elements shifts, and the loop continues to the next element in the modified list, which is not the expected one.

To resolve this issue and accurately iterate over the list, we can use list comprehensions or in-place alterations. List comprehensions allow us to create a new list with only the elements that meet the specified condition:

numbers = [n for n in numbers if n >= 20]

Alternatively, in-place alteration can be performed using slice-assignment:

numbers[:] = (n for n in numbers if n >= 20)

By modifying the list in this manner, the iteration is not affected and the desired result can be achieved.

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