Question 1

What is a race condition and how do locks prevent it?

Accepted Answer

**Target Audience:** Developers starting with threads who want to understand why shared data goes wrong — no prior concurrency knowledge needed.
**Difficulty:** Medium
**Python version:** Bytecode and behaviour checked on CPython 3.11, 3.12 and 3.13 (standard GIL builds); 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 exp

Question 2

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 3

What is composition versus inheritance?

Accepted Answer

**Target Audience:** Developers who can write classes and subclasses and want to know how to choose between the two main ways of reusing code.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11.17, 3.12 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output shown was produced

Question 4

When do you use threading, multiprocessing, or asyncio?

Accepted Answer

**Target Audience:** Developers who know the words thread, process and async but want a reliable way to pick one — no prior concurrency experience needed.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13 (standard GIL builds); 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 expect

Question 5

How do ThreadPoolExecutor and ProcessPoolExecutor work?

Accepted Answer

**Target Audience:** Developers who have used a loop and a function, and want to run that function many times at once safely — no prior concurrency knowledge needed.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13 (standard GIL builds); 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 intervi

Question 6

When should you still use multiprocessing vs free-threaded threads in Python 3.12+?

Accepted Answer

**Target Audience:** Developers who know threads, processes and the GIL at a basic level and must choose a concurrency model for CPU-bound Python work.
**Difficulty:** Medium
**Python version:** Example 1 runs in the page on Pyodide 0.25.1 (CPython 3.11.3) and was checked on CPython 3.11. Example 2 starts threads and processes, so it was run on CPython 3.13 (default build). **We did not have a free-threaded interpreter**, so performance claims about it come from the CPython docs and PEPs cited i

Question 7

What is the difference between concurrency and parallelism in Python?

Accepted Answer

**Target Audience:** Python developers who have heard of threads and asyncio but mix up the two words — no prior concurrency knowledge needed.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13 (standard GIL builds); 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 out

Question 8

What is pickling and what are its risks?

Accepted Answer

**Target Audience:** Python developers who know the basics and want to understand serialisation and its security model
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13 (and in the page's Pyodide 3.11 runtime)

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every output block in the **Verified** section was copied from a real run o

Question 9

What is Python 3.12's free-threaded (no-GIL) build and what changes?

Accepted Answer

**Target Audience:** Python developers who know what a thread is and have heard of the GIL, and want accurate version facts and the practical consequences for their code.
**Difficulty:** Hard
**Python version:** Example 1 runs in the page on Pyodide 0.25.1 (CPython 3.11.3) and was also run on CPython 3.11, 3.12 and 3.13. Example 2 was run on the default (GIL) build of CPython 3.13. **We did not have a free-threaded interpreter available**, so statements about how `python3.13t` / `python3.14t` be

PYTHONDECODED.

Medium