How to Safely Remove Elements from a List While Iterating in Python?

Removing Elements from a List During Iteration

When iterating over a list in Python, it's important to know how to remove items that meet specific criteria. However, attempting to modify a list while iterating can raise issues.

The Dilemma

Consider the following code block:

for tup in somelist:
    if determine(tup):
         # How do I remove 'tup' here?

How can you remove tup from the list while preserving the iterator?

Solutions

There are several solutions to this problem:

1. List Comprehension

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

This method creates a new list containing only the elements that don't need to be removed.

2. Slice Assignment

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

This assignment mutates the existing list to only contain the desired elements.

3. Itertools

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

This approach uses Python's filterfalse function to filter out unwanted elements.

Additional Notes

• The determine function is a placeholder for your custom criteria function that determines which elements to remove.
• The slice assignment solution can be useful if other references to somelist need to reflect the changes.
• If you only need to check for a specific value, you can use if statements directly in the loop.

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