What do the global and nonlocal keywords do? | Interview Question

Q: What do the global and nonlocal keywords do?

**Target Audience:** Python beginners and intermediate developers preparing for interviews — you should know what a function and a nested function are. **Difficulty:** Medium **Python version:** Behaviour and error messages 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 Picture a shared office whiteboard in the hallway and a small notepad on your own desk. When you are at your desk and write "meeting at 3", it goes on **your notepad** by default — you do not walk out and scribble on the hallway board just because it also has a "meeting" line. You can still **read** the hallway board any time. If you really want to change th

The problem

Question presented to candidate: "Inside a function, how do you change a variable that was defined outside it? Explain what global and nonlocal do and when you would — or would not — use them."

What a strong answer should cover:

Assignment inside a function binds a local name by default; global x makes assignments rebind the module-level x, and nonlocal x rebinds x in the nearest enclosing function.
The difference between rebinding a name (needs a keyword) and mutating the object it points to (items.append(1) needs none); x += 1 is a rebinding.
nonlocal requires an existing binding in an enclosing function (never the module) and is checked at compile time; global can even create a new module name.
They are declarations for the whole function body: using the name before the declaration is a SyntaxError, and a parameter cannot also be declared global or nonlocal.
Design trade-offs: global hides shared state and is not thread safe on its own; prefer parameters and return values, a class, or a closure with nonlocal for private state.

Code / implementation expected: Optional — a counter with nonlocal, or spotting the UnboundLocalError in a function that does total += n.

global
nonlocal

Try it yourself

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Solution

Target Audience: Python beginners and intermediate developers preparing for interviews — you should know what a function and a nested function are. Difficulty: Medium Python version: Behaviour and error messages 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

Picture a shared office whiteboard in the hallway and a small notepad on your own desk. When you are at your desk and write "meeting at 3", it goes on your notepad by default — you do not walk out and scribble on the hallway board just because it also has

The solution is hidden so you can try first

9 min read · 11 sections

Run the code

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

Pythonglobal vs nonlocal, and mutation vs rebinding

Practice

PythonThe compile-time rules, and the bytecode each keyword produces

Practice

Discussion

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