MediumDSA9 min read

What is the late-binding closure pitfall in loops?

Q: What is the late-binding closure pitfall in loops?

**Target Audience:** Python beginners and intermediate developers preparing for interviews — you should know what a lambda and a list comprehension are. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13; the examples run in the page on Pyodide 0.25.1 (CPython 3.11.3). > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output shown was produced by actually running the code — see the **Verified** section. ## 1. Why This Even Matters — A Story First A teacher hands three pupils a sticky note each that says "your number is whatever is on the board". She writes 0 on the board while giving out the first note, 1 for the second and 2 for the third. At the end of the day each pupil reads their note, looks at the board — and all three see 2. Nobody got the number that was on the board when they received the note; they all got *

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The problem

Question presented to candidate: "What does [f() for f in [lambda: i for i in range(3)]] return, and why? How would you fix it?"

What a strong answer should cover:

The output is [2, 2, 2]: each lambda captures the variable i, not its value, and reads it when called, after the loop has finished.
The mechanism: all the functions share one cell for i (provable with __closure__); at module level there is no cell at all, they read the global i.
It affects any function created in a loop (def handlers, callbacks), not only lambdas and comprehensions.
Fixes: a default argument lambda i=i: i, a factory function that takes i as a parameter, or functools.partial; trade-offs of each (a default can be overridden by callers).
A default argument snapshots the object, not its contents: capturing a mutable list still shares it.

Code / implementation expected: Yes — predict the output, then show at least one fix and explain why it works.

closures
late-binding

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: Python beginners and intermediate developers preparing for interviews — you should know what a lambda and a list comprehension are. Difficulty: Medium Python version: Behaviour checked on CPython 3.11, 3.12 and 3.13; the examples run in the page on Pyodide 0.25.1 (CPython 3.11.3).

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

1. Why This Even Matters — A Story First

A teacher hands three pupils a sticky note each that says "your number is whatever is on the board". She writes 0 on the board while giving out the first note, 1 for the second and 2 for the third. At the end of the day each pupil reads their note, looks at the board — and all