Question 1

How does collections.deque work internally?

Accepted Answer

**Target Audience:** Developers who use `deque` as "the fast queue" and want to explain why, up to candidates expected to describe the CPython data structure.
**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); the block-growth numbers were identical on python3.13.

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

Question 2

What is the @property decorator?

Accepted Answer

**Target Audience:** Python beginners who know classes, up to candidates expected to explain descriptors
**Difficulty:** Medium
**Python version:** behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime); error messages also checked on 3.12 and 3.13

> **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

Question 3

How does heapq work in Python?

Accepted Answer

**Target Audience:** Developers who have heard "use a heap for a priority queue" and want to know what `heapq` really does, up to candidates expected to explain the algorithm and its costs.
**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). The 3.14 max-heap functions are cited from the docs, not run.

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

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 does an LRU cache work internally?

Accepted Answer

**Target Audience:** Candidates facing the classic "design an LRU cache" question, and anyone who uses `functools.lru_cache` and wants to know what it does inside.
**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

Question 6

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 7

How does the bisect module work?

Accepted Answer

**Target Audience:** Beginners who know lists and sorting; candidates who want to write binary search confidently in an interview.
**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). `key=` needs Python 3.10 or newer.

> **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 o

Question 8

Why does Python cache small integers?

Accepted Answer

**Target Audience:** Python developers who know what a variable is and have seen `is` and `==`. No knowledge of CPython's C code is needed.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11 (64-bit), with 3.12 and 3.13 where they differ. Examples 1 and 2 also run in the page on Pyodide 0.25.1 (CPython 3.11.3); Example 3 needs 3.12+.

> **How to read this doc:** Each idea is explained in plain words first. When a word appears that an interviewer expects you to know, it

Question 9

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 10

What is EAFP vs LBYL in Python?

Accepted Answer

**Target Audience:** Python developers who know try/except and want to know when to check first and when to just try
**Difficulty:** Medium
**Python version:** Checked on CPython 3.11 (and in this page on Pyodide 0.25.1, 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 output block in the **Verified** section was copied from a real run of the e

PYTHONDECODED.

Medium