Question 1

What does if __name__ == "__main__" do?

Accepted Answer

**Target Audience:** Beginners writing their first scripts and modules; anyone who has copied the line without knowing why.
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11.17, 3.12.3 and 3.13.16 (identical output) and in the page runner (Pyodide 0.25.1, CPython 3.11.3).

> **How to read this doc:** Ideas come in plain words first, then a `📌 Interview term:` callout with the precise name an interviewer uses. Every output below is copied from a real run; the **Verified** section

Question 2

What is uv and why is it replacing pip/poetry in 2025-26?

Accepted Answer

**Target Audience:** Python developers who know pip and venv and keep hearing about uv, and candidates asked about modern Python tooling
**Difficulty:** Easy
**Python version:** The in-page examples were run on CPython 3.11 and in this page (Pyodide 0.25.1, CPython 3.11.3). The shell sessions were run with uv 0.11.32 on Linux against python3.11; uv changes quickly, so check `uv --version` and the docs for your release.

> **How to read this doc:** Each idea is explained in plain language first,

Question 3

What do the global and nonlocal keywords do?

Accepted Answer

**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.

Question 4

What is the Global Interpreter Lock (GIL)?

Accepted Answer

**Target Audience:** Python developers who have used or heard of threads and want to understand what CPython actually allows. No operating-systems background is assumed.
**Difficulty:** Medium
**Python version:** Behaviour checked on standard (GIL) builds of CPython 3.11, 3.12 and 3.13. Example 1 also runs in the page on Pyodide 0.25.1 (CPython 3.11.3); Example 2 uses real threads, which the browser build cannot start, so it is verified on 3.12.

> **How to read this doc:** Each idea is explaine

Question 5

What is the LEGB rule for scope resolution?

Accepted Answer

**Target Audience:** Python beginners and intermediate developers preparing for interviews — you only need to know what a 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

Question 6

What is a deadlock in Python?

Accepted Answer

**Target Audience:** Developers who use locks or threads and want to understand why programs freeze — no prior concurrency knowledge needed beyond what a lock is.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13; the in-page example runs 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 outpu

Question 7

What is the difference between the call stack and the heap in Python?

Accepted Answer

**Target Audience:** Developers coming from C, C++, Java or Go who want to map "stack vs heap" onto Python, and Python developers debugging recursion or memory
**Difficulty:** Medium
**Python version:** Checked on CPython 3.11, 3.12 and 3.13 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); all gave identical output

> **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 expect

Question 8

How does functools.lru_cache work?

Accepted Answer

**Target Audience:** Python developers who know basic decorators, up to candidates asked about caching internals
**Difficulty:** Medium
**Python version:** behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime)

> **How to read this doc:** Every idea is first explained in plain words. When a term has a precise name an interviewer expects to hear, it is introduced as a 📌 **Interview term** callout. The Quick Glossary at the end lists all of them.

## 1. Why This Ev

Question 9

What is the late-binding closure pitfall in loops?

Accepted Answer

**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 show

Question 10

What is monkey patching in Python?

Accepted Answer

**Target Audience:** Developers who have seen `mock.patch` in tests or a "quick fix" that reassigns a library function, up to candidates who must explain scope, the import trap and the risks.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3); spot checks on 3.12 and 3.13 are noted.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:

PYTHONDECODED.

Easy

What does if name == "main" do?