Remove items from a Python list while iterating it

This post will discuss how to remove items from a Python List while iterating it.

It is not recommended removing items from a list within a loop as this might cause the list to skip a few items and lead to incorrect output. This happens because removing items from the list while iterating changes the index and its length.

if __name__ == '__main__':

nums = [1, 2, 3, 4, 5]

for x in nums:

nums.remove(x)

print(nums)

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This post provides an overview of some available alternatives to avoid this non-deterministic behavior.

1. Using List Comprehension

You can use a list comprehension to create a new list containing only the desired items:

if __name__ == '__main__':

nums = [1, 2, 3, 4, 5]

odd = [x for x in nums if x & 1]

print(odd)

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This works fine. However, it does not modify the input list in-place but creates a new list instead. This might cause issues when there are other references to the list which need updating. Fortunately, in Python, you can simply alter the list in-place by assigning to the slice [:].

if __name__ == '__main__':

nums = [1, 2, 3, 4, 5]

nums[:] = [x for x in nums if x & 1]

print(nums)

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2. Using `itertools` module

In Python 3, you could also use the itertools module instead of list comprehension. It has a filterfalse() function, which filters elements from the list for which the predicate is False.

from itertools import filterfalse

def iseven(x):

return x & 1 == 0

if __name__ == '__main__':

nums = [1, 2, 3, 4, 5]

nums[:] = filterfalse(iseven, nums)

print(nums)

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3. Creating Copy of List

Here, the idea is to iterate over a copy of the list and then modify the original list as required. Here’s a working example:

if __name__ == '__main__':

nums = [1, 2, 3, 4, 5]

for x in list(nums):

if x & 1 == 0:

nums.remove(x)

print(nums)

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4. Iterate Backwards

Another plausible way is to iterate backward in the list and remove the elements from it. This way, nothing is skipped. This approach has a major advantage over the other approaches as it does not create any copies of the list, and does the job in a single pass and in-place.

if __name__ == '__main__':

nums = [1, 2, 3, 4, 5]

for x in reversed(range(len(nums))):

if x & 1 == 0:

nums.remove(nums[x])

print(nums)

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That’s all about removing items from a Python list while iterating it.

Remove items from a Python list while iterating it

1. Using List Comprehension

2. Using itertools module

3. Creating Copy of List

4. Iterate Backwards

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2. Using `itertools` module