Mutable default argument (ARCH2194)

Key python.quality.mutable-default-arg · Severity warning · Category quality

Python default arguments are evaluated once at definition time; a mutable default is shared across every call, causing a classic bug. Use None instead.

Incorrect

def add_item(item, bucket=[]):
    bucket.append(item)
    return bucket

Correct

def add_item(item, bucket=None):
    if bucket is None:
        bucket = []
    bucket.append(item)
    return bucket

Fix

Default to None and create the new mutable object inside the body.