How do closures capture variables in Python? | Interview Question

Q: How do closures capture variables in Python?

**Target Audience:** Python beginners and intermediate developers preparing for interviews — you should know what a nested function is. **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 bakery owner hires a new delivery driver and, before going home, hands over a backpack containing the shop key and a note: "today's discount: 10%". The owner leaves; the driver keeps working all evening, using the key and the note from the backpack. Now the twist: the backpack is shared. If the owner phones in and changes the note to "20%", the driver sees 20% at the next delivery — the dri

The problem

Question presented to candidate: "What is a closure in Python, and what exactly does the inner function capture — the value of the outer variable, or something else? Show me how you would prove it."

What a strong answer should cover:

A closure is a function that keeps access to variables of the enclosing function after that function has returned.
Python captures the variable, not a snapshot of its value: the variable lives in a cell object shared by the outer function and every inner function that uses it.
The inspection points: outer.__code__.co_cellvars, inner.__code__.co_freevars, inner.__closure__ and cell.cell_contents; a function with no free variables has __closure__ of None.
Reading a captured variable needs nothing; rebinding it needs nonlocal. Each call of the outer function makes new cells.
Globals are not captured at all, and late binding means the value is read when the inner function runs — the source of the loop pitfall.
Real uses: function factories, decorators, callbacks and small private state; the closure keeps the captured objects alive.

Code / implementation expected: Yes — a factory or counter closure, ideally with __closure__ inspected to show the shared cell.

closures
cells

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Solution

Target Audience: Python beginners and intermediate developers preparing for interviews — you should know what a nested function is. 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 bakery owner hires a new delivery driver and, before going home, hands over a backpack containing the shop key and a note: "today's discount: 10%". The owner leaves; the driver keeps working all evening, using the key and the note from the backpack. Now the twist: the backpack is shared. If t

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10 min read · 12 sections

Run the code

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

PythonLook inside a closure: cells, co_freevars and __closure__

Practice

PythonClosures at work: factories, a memoizing decorator, lifetime

Practice

Discussion

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