How do default, keyword-only and positional-only parameters work?

Q: How do default, keyword-only and positional-only parameters work?

**Target Audience:** Beginners to intermediate — you can write functions and have seen `*args`. **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 `SyntaxError` wording was also checked on 3.12.3 and 3.13.16. > **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 Think of a paper form at a bank. A few boxes at the top have no labels: "first box is your account number, second box is the branch". You fill them by **position** — nobody cares what the bank internally calls them. Further down the form there are boxes that must be ticked **by label**: "Send paper statements? ☐ yes". Nobody wants to fill that one by counting boxes. And many

The problem

Question presented to candidate: "Walk me through this signature: def connect(host, port=5432, /, timeout=10, *, retries=3). What do the slash and the star mean, how are the defaults stored, and why would an API designer choose positional-only or keyword-only parameters?"

What a strong answer should cover:

The five parameter kinds: positional-only (before /, Python 3.8+, PEP 570), positional-or-keyword, *args, keyword-only (after * or *args, Python 3.0+, PEP 3102), **kwargs
Defaults are evaluated once when def runs and stored on the function (__defaults__, __kwdefaults__); that is why mutable defaults are shared
Ordering rules: a parameter without a default cannot follow one with a default among positional parameters, but keyword-only parameters can be required (no default) in any order
Why keyword-only: readable call sites for flags and options (retries=3 instead of a mystery 3); why positional-only: the name is not part of the API, so it can be renamed, and the name stays free for **kwargs, matching builtins like len() and divmod()
The exact TypeError messages for each misuse, and that inspect.signature / Signature.bind show how Python maps arguments to parameters

Code / implementation expected: Yes — write a signature that uses / and *, and show which calls succeed and which raise TypeError.

functions
parameters

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 and have seen *args. 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 SyntaxError wording was also checked on 3.12.3 and 3.13.16.

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

Think of a paper form at a bank. A few boxes at the top have no labels: "first box is your account number, second box is the branch". You fill them by position — nobody cares what the bank internally calls them. Further down the form there are boxes that must be ticked **b

The solution is hidden so you can try first

9 min read · 10 sections

Run the code

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

PythonThe slash and the star: which calls work and which raise

Practice

PythonWhere defaults live: __defaults__, __kwdefaults__, the code object and inspect