Question 1

How do you sort with a custom key, and how does sorted differ from sort?

Accepted Answer

**Target Audience:** Beginners to intermediate Python developers — you should know lists, tuples and simple functions.
**Difficulty:** Easy
**Python version:** Behaviour checked on CPython 3.11; the examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

> **How to read this doc:** Each idea comes in plain language first. Words an interviewer expects you to know are tagged `📌 Interview term:` and defined on the spot; the Quick Glossary at the end lists them all. Every output and comp

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

Why is using a mutable default argument dangerous?

Accepted Answer

**Target Audience:** Beginners to intermediate — you can write functions with default values.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); the dataclass error message was also checked on 3.12.3.

> **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 by actual

Question 4

What are decorators in Python?

Accepted Answer

**Target Audience:** Python beginners who have written functions, up to candidates preparing for mid-level interviews
**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 Th

Question 5

What are lambda functions and map/filter/reduce?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — you know lists, loops and basic functions.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11.17 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 by actually running the code; the **Verified** sect

Question 6

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 7

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 8

What is a decorator factory (a decorator with arguments)?

Accepted Answer

**Target Audience:** Developers who can write a plain decorator and want configurable ones
**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 Even Matters — A Story F

Question 9

How does functools.reduce fold a sequence?

Accepted Answer

**Target Audience:** Developers meeting `reduce` for the first time, or coming from JavaScript `Array.prototype.reduce` or functional languages; candidates expected to trace a fold by hand.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); the keyword-argument check was also run on 3.12.3 and 3.13.16.

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

Question 10

Why use functools.wraps when writing decorators?

Accepted Answer

**Target Audience:** Developers who can write a basic decorator and want to know why the extra line matters
**Difficulty:** Medium
**Python version:** behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime); the 3.12 and 3.13 attribute list was printed on those interpreters

> **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

PYTHONDECODED.

Easy

How do you sort with a custom key, and how does sorted differ from sort?

Medium

What is the @property decorator?

Why is using a mutable default argument dangerous?

What are decorators in Python?

What are lambda functions and map/filter/reduce?

How does an LRU cache work internally?

How does functools.lru_cache work?

What is a decorator factory (a decorator with arguments)?

How does functools.reduce fold a sequence?

Hard

What is the descriptor protocol?