A function parameter that defaults to `[]`, `{}`, or `set()`. Python evaluates the default once, so every call that omits the argument shares and mutates the same object.
Each example shows the exact code shape the rule is looking for, then the smallest version that keeps the intent without hiding risk or adding noise.
def add_tag(tag, tags=[]):
tags.append(tag)
return tags
add_tag("a") # ["a"]
add_tag("b") # ["a", "b"]: the default list is sharedDeterministic signature parse: reads each `def` signature, including multi-line ones, with strings and comments masked, and flags a parameter whose default is `[]`, `{}`, or `set()`.
def add_tag(tag, tags=None):
tags = tags if tags is not None else []
tags.append(tag)
return tagsDefault to `None` and create the value inside the body: `items = items if items is not None else []`.
Agents write the default they want each call to start with, which is how defaults behave in many other languages. In Python it leaks state between calls.
ai-slop/python-mutable-default
ai-slop
Deterministic signature parse: reads each `def` signature, including multi-line ones, with strings and comments masked, and flags a parameter whose default is `[]`, `{}`, or `set()`.
`None` defaults, immutable defaults such as tuples, and empty literals nested inside a call (for example FastAPI's `Query(default=[])`) are not flagged.
Part of aislop's Python rule set, scored in the strict tier. v0.15.0 stopped flagging FastAPI, typer, and pydantic marker defaults.