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 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 3

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 4

What does the functools module offer?

Accepted Answer

**Target Audience:** Python developers who write decorators, sort objects or cache expensive calls; candidates asked the open-ended "what is in functools?".
**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:** Each idea is explained in plain words first, then tagged with `📌 Interview term:` — that is the exact vocabulary an interviewer expects to hear. Every output shown was printed

Question 5

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 6

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 7

How does multiprocessing share data between processes?

Accepted Answer

**Target Audience:** Python developers who have used threads or `concurrent.futures` and want to know what really happens to their data when work moves to another process.
**Difficulty:** Hard
**Python version:** The pickling example runs in the page on Pyodide 0.25.1 (CPython 3.11.3) and was checked on CPython 3.11. The process examples need real processes, so they were run on CPython 3.12 (and gave identical output on 3.13) on Linux.

> **How to read this doc:** Every idea comes in plain langu

PYTHONDECODED.

Easy