How does iterable unpacking work?

Q: How does iterable unpacking work?

**Target Audience:** Beginners to intermediate Python developers — you should know what a list and a tuple are. **Difficulty:** Easy **Python version:** Behaviour and `dis` output checked on CPython 3.11 (3.12 where noted); the examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3). > **How to read this doc:** Ideas come in plain language first. Words an interviewer expects you to use are tagged `📌 Interview term:` and defined on the spot, then listed again in the Quick Glossary. All output shown is from real runs. ## 1. Why This Even Matters — A Story First You come home with a bag of shopping: milk, bread, eggs. You do not carry the bag to the fridge and then reach into it three times, saying "item number 0 goes on the top shelf, item number 1 ...". You open the bag and put each thing straight where it belongs, in one movement. Iterable unpacking is that movement in code. Instead of ~~~python point = (3, 4) x = point[0] y = point[1] ~~~ you write `x, y = point`. Pyth

The problem

Question presented to candidate: "Explain what happens in a line like first, *rest = items or a, b = b, a. How does iterable unpacking work in Python, what are the rules, and where else do you see the star syntax?"

What a strong answer should cover:

The right side is evaluated completely first, then its items are assigned to the targets on the left, from left to right; any iterable works (list, tuple, string, generator, dict keys).
The count must match, otherwise ValueError: too many values to unpack or not enough values to unpack.
One starred target (PEP 3132) collects the leftovers into a list, possibly empty; two stars are a SyntaxError.
Nested targets mirror nested data: name, (lat, lng) = record; it is used in for headers, enumerate, dict.items().
The other direction (PEP 448, Python 3.5): [*a, *b], {**d1, **d2}, f(*args, **kwargs).
A CPython detail: a, b = b, a compiles to a SWAP instruction on 3.11, no tuple is built; longer targets use UNPACK_SEQUENCE / UNPACK_EX.

Code / implementation expected: Yes — a few short assignments showing a starred target, a swap, a nested target and the error case.

unpacking

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 Python developers — you should know what a list and a tuple are. Difficulty: Easy Python version: Behaviour and dis output checked on CPython 3.11 (3.12 where noted); the examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

How to read this doc: Ideas come in plain language first. Words an interviewer expects you to use are tagged 📌 Interview term: and defined on the spot, then listed again in the Quick Glossary. All output shown is from real runs.

1. Why This Even Matters — A Story First

You come home with a bag of shopping: milk, bread, eggs. You do not carry the bag to the fridge and then reach into it three times, saying "item number 0 goes on the top shelf, item number 1 ...". You open the bag and put each thing straight where it belongs, in one movement.

Iterable unpacking is that movement in code

The solution is hidden so you can try first

10 min read · 11 sections

Run the code

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

PythonUnpacking forms: basic, starred, swap, nested, loops and spreading

Practice

PythonErrors and gotchas: count mismatch, two stars, assignment order, trailing comma