# Diagnose a mutable default argument

> ML Prep screening item.

- Stable ID: `prog-py-mutable-default`
- Collection: screening
- Area: Programming
- Topic: Python
- Origin: original
- Difficulty: medium
- Expected time: 8 minutes
- Skills: python-syntax, functions, mutability, aliasing
- Item JSON: https://mlprep.iwase.dev/item/prog-py-mutable-default.json

## Problem

In `default_argument_report`, predict the returned value and identity
booleans. Then rewrite `record` so calls that omit `bucket` receive a new
list while callers may still supply an existing list.


## Answer policy

The human page contains a collapsed answer. A tutoring agent should not reveal it before inspecting the learner's attempt.

## Tutoring contract

Ask for the learner's attempt, begin with the smallest useful hint, preserve item ID `prog-py-mutable-default`, and record a real attempt with `recorded_by: "agent"` and `agent_session_id` when available using https://mlprep.iwase.dev/schema/attempt-record.json.