Why is using a mutable default argument dangerous?

Q: Why is using a mutable default argument dangerous?

**Target Audience:** Beginners to intermediate — you can write functions with default values. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); the dataclass error message was also checked on 3.12.3. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First A hotel puts a guest book on the reception desk. The idea is "if a guest does not bring their own notebook, give them the desk one". Sounds harmless — until you notice the hotel bought **one** desk notebook on opening day. Every guest without a notebook writes into the same book, and the third guest finds the first two guests' notes already in it. A mutable default argument is that des

The problem

Question presented to candidate: "Here is def add_item(item, bucket=[]): bucket.append(item); return bucket. Call it twice without a bucket. What do you get, why, and how would you fix it?"

What a strong answer should cover:

Default expressions are evaluated once, when the def statement executes, not on every call; the resulting object is stored on the function in __defaults__ (or __kwdefaults__)
So a mutable default ([], {}, set(), a custom object) is one shared object; mutations from one call leak into every later call that relies on the default
The fix: None as a sentinel and create the object inside the body; a private object() sentinel when None is a valid input; field(default_factory=list) in dataclasses
The same "evaluated once" rule explains def f(ts=datetime.now()) freezing a timestamp, and the class-attribute version of the bug
Linters catch it (Ruff / flake8-bugbear B006, Pylint W0102 dangerous-default-value), and the deliberate exception: a default dict used as a cache, usually better done with functools.lru_cache

Code / implementation expected: Yes — reproduce the bug, show __defaults__ changing, then write the None-sentinel version.

mutable-default

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners to intermediate — you can write functions with default values. Difficulty: Medium Python version: Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); the dataclass error message was also checked on 3.12.3.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

A hotel puts a guest book on the reception desk. The idea is "if a guest does not bring their own notebook, give them the desk one". Sounds harmless — until you notice the hotel bought one desk notebook on opening day. Every guest without a notebook writes into the same book, and the

The solution is hidden so you can try first

8 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonThe shared default list, seen through __defaults__, and the sentinel fixes

Practice

PythonSame rule elsewhere: def-time evaluation, dicts, class attributes, dataclasses, a deliberate cache