Question 1

What are mutable and immutable types in Python?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — no prior Python knowledge assumed.
**Difficulty:** Easy
**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 output shown in the **Verified** section was copied from a real run o

Question 2

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 3

How do you measure and profile Python performance?

Accepted Answer

**Target Audience:** Developers who have heard "profile before you optimise" but never actually read a profiler table; no prior performance work assumed.
**Difficulty:** Medium
**Python version:** Examples run on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, CPython 3.11.3). The command-line profiles were run on python3.11 and python3.12 on our machine; their timings are from that run only.

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

Question 4

What is the difference between __new__ and __init__?

Accepted Answer

**Target Audience:** Python developers who write classes with `__init__` and have seen `__new__` in library code or an interview question.
**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 by a

Question 5

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 6

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 7

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 8

What is a metaclass?

Accepted Answer

**Target Audience:** Python developers who are comfortable with classes and want to know what sits one level above them
**Difficulty:** Hard
**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); the hook order is identical on all of them

> **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

Question 9

What is the descriptor protocol?

Accepted Answer

**Target Audience:** Intermediate Python developers who use `@property` and methods every day and want to know the machinery under them.
**Difficulty:** Hard
**Python version:** Behaviour checked on CPython 3.11 (outputs identical on 3.12 and 3.13); both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

> **How to read this doc:** Plain-language explanation comes first. The vocabulary an interviewer expects is tagged `📌 Interview term:` and defined on the spot; all terms are co

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