loop-iterator-mutation (B909)

Derived from the flake8-bugbear linter.

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Checks for mutations to an iterable during a loop iteration.

When iterating over an iterable, mutating the iterable can lead to unexpected behavior, like skipping elements or infinite loops.

items = [1, 2, 3]

for item in items:
    print(item)

    # Create an infinite loop by appending to the list.
    items.append(item)