PERF401 is triggered on code that has the walrus operator inside. The walrus operator is not available in the list-comprehension if-shorthand and it can not be recreated within a single list-comprehension.

Minimal example

items = []

for i in range(5):
    if j := i:
        items.append(j)

This code can obviously be refactored to not include the walrus operator but there are more complex scenarios where it is impossible to do so without using multiple list-comprehensions or duplicating code.

More complex example

import re

items = []

for s in ["abc", "cde", "efg"]:
    if match := re.match(r"e[a-z]+", s):
        items.append(match.group(0))