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

How do you run external commands with subprocess safely?

Accepted Answer

**Target Audience:** Developers who have used `os.system` or copied a `subprocess` line from the internet, and want to know what each argument does and why it matters.
**Difficulty:** Medium
**Python version:** Example 1 (`shlex`) was run on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, CPython 3.11.3). Examples 2 and 3 start real processes, which a browser runtime cannot do, so they are shown with an "Open in playground" button and were run on python3.12 (identical output on

Question 3

How does exception propagation work up the call stack?

Accepted Answer

**Target Audience:** Python developers who can write try/except and want to know exactly what happens between a raise and its handler
**Difficulty:** Medium
**Python version:** Checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); the thread and process example on CPython 3.12

> **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 bloc

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

What is asyncio.TaskGroup and why does it replace gather?

Accepted Answer

**Target Audience:** Developers who have written `asyncio` code with `gather` or `create_task` and want to know what TaskGroup changes and when to pick which.
**Difficulty:** Medium
**Python version:** `TaskGroup` needs Python 3.11+. Behaviour checked on CPython 3.11, 3.12 and 3.13 (same output); both examples also run in this page on Pyodide 0.25.1 (CPython 3.11.3).

> **How to read this doc:** Every idea comes in plain language first. Words an interviewer expects you to use are tagged `📌 Inte

Question 6

How does yield from delegation work?

Accepted Answer

**Target Audience:** Python developers who know what a generator is and want to understand delegation properly; interview candidates who will be asked "isn't it just a loop?".
**Difficulty:** Medium
**Python version:** All behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); the bytecode comparison was also run on CPython 3.12.3.

> **How to read this doc:** Each idea is explained in plain words first, then tagged with `📌 Interview term:` — that is the e

Question 7

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 8

How do you raise and split ExceptionGroups manually?

Accepted Answer

**Target Audience:** Python developers who have seen `ExceptionGroup` come out of a `TaskGroup` and want to create, inspect and split groups themselves.
**Difficulty:** Medium
**Python version:** Python 3.11+ (PEP 654). Behaviour checked on CPython 3.11, 3.12 and 3.13 (identical output); both examples run in this page on Pyodide 0.25.1 (CPython 3.11.3).

> **How to read this doc:** Every idea comes in plain language first. Words an interviewer expects you to use are tagged `📌 Interview term:` a

Question 9

What is a context manager?

Accepted Answer

**Target Audience:** Python learners who use `with open(...)` without knowing what it does, up to candidates who need the exact protocol and CPython details.
**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 where made.

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

Question 10

How do you use multiple context managers at once?

Accepted Answer

**Target Audience:** Developers who know the single `with` statement and want to combine several resources correctly, including candidates who need the edge cases.
**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).

> **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

PYTHONDECODED.

Medium

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

How do you run external commands with subprocess safely?

How does exception propagation work up the call stack?

When do you use threading, multiprocessing, or asyncio?

What is asyncio.TaskGroup and why does it replace gather?

How does yield from delegation work?

What is a deadlock in Python?

How do you raise and split ExceptionGroups manually?

Hard

What is an async iterator and async for?

How does TaskGroup handle cancellation and ExceptionGroup?

What is TypeVarTuple and Unpack for variadic generics?

What is the difference between a generator and a coroutine?

How does the asyncio event loop schedule coroutines?

How does async/await work in Python?