How Can I Safely Remove Items from a List While Iterating in Python?

Removal of Items During List Iteration

When iterating over a list in Python, it can become necessary to remove certain items based on specific criteria. However, attempting to remove an item directly within the iteration can result in unexpected behavior. This is where the need for alternative methods arises.

List Comprehension

One solution is to employ list comprehension, which allows for the creation of a new list containing only the desired elements. For example:

somelist = [x for x in somelist if not determine(x)]

This comprehension creates a new list that includes all elements of somelist where the determine() function evaluates to False.

Mutation with Slicing

Another approach is to mutate the existing list using slices. By assigning to somelist[:], the list can be modified to contain only the acceptable elements:

somelist[:] = [x for x in somelist if not determine(x)]

This method retains the original list reference, making it useful when other parts of the program need to access the modified list.

Itertools

In Python 2 or 3, itertools can be utilized to achieve the same result. In Python 2:

from itertools import ifilterfalse
somelist[:] = ifilterfalse(determine, somelist)

In Python 3:

from itertools import filterfalse
somelist[:] = filterfalse(determine, somelist)

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