Why Does Modifying a List During Iteration Produce Unexpected Results?

Unexpected Results While Modifying a List During Iteration

When modifying a list while iterating over it, unexpected results can occur. Consider the following code:

numbers = list(range(1, 50))

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

print(numbers)

Despite the intention to remove numbers less than 20 from the list, the resulting output includes some of these numbers. This discrepancy arises because the list is being altered during iteration.

Specifically, after removing the first number below 20 (1), the loop proceeds to the next item in the original list, which is now 3 instead of 2. This process continues, resulting in the removal of all odd numbers below 20.

To avoid this issue, it's recommended to use an alternative approach, such as list comprehensions or generator expressions. Here's an example using a list comprehension:

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

This code preserves the original order of the list while filtering out unwanted elements. Alternatively, you can use in-place alteration with a generator expression:

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

If you need to perform operations on an element before removing it, consider using an indexed loop and setting the element to None. Then, create a new list that includes only non-None elements:

for i, n in enumerate(numbers):
    if n < 20:
        print("do something")
        numbers[i] = None
numbers = [n for n in numbers if n is not None]

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