Question 1

What are mixins in Python?

Accepted Answer

**Target Audience:** Developers who know single inheritance and want to use (and explain) multiple inheritance the way Python libraries do.
**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

Question 2

What is the difference between class variables and instance variables?

Accepted Answer

**Target Audience:** Beginners who have written a class with `__init__`, up to intermediate developers revising Python's object model for interviews.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11 and 3.13 (identical output); every example also runs 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. When a word appears that an interviewer expects you to know, it is tagged with `📌 Interview te

Question 3

What is Python 3.12's new type parameter syntax (type, generic classes)?

Accepted Answer

**Target Audience:** Python developers who know `TypeVar` and `Generic` and are moving a codebase to Python 3.12 or later.
**Difficulty:** Medium
**Python version:** The syntax needs **Python 3.12+** (defaults need 3.13+). Examples 2 and 3 were run on CPython 3.12.3 and 3.13.16 and are shown as code here, because the page runner is Pyodide 0.25.1 (CPython 3.11.3), where the syntax is a `SyntaxError`. Example 1 runs in the page. Type-checker output is from mypy 2.3.1.

> **How to read this doc:**

Question 4

What is the Method Resolution Order (MRO)?

Accepted Answer

**Target Audience:** Python developers who have used inheritance but never had to think about the order of lookup, up to candidates who need to explain C3 and `super()` precisely.
**Difficulty:** Hard
**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), with the MRO error message also checked on 3.13.

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

Question 5

How does attribute lookup work on a Python object?

Accepted Answer

**Target Audience:** Intermediate Python developers who know what a class, an instance and a method are, and want the exact rules an interviewer probes.
**Difficulty:** Hard
**Python version:** Behaviour and bytecode checked on CPython 3.11 (and 3.12/3.13 where noted); both examples also run in the 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:` and defined on t

PYTHONDECODED.

Medium

What are mixins in Python?

What is the difference between class variables and instance variables?

What is Python 3.12's new type parameter syntax (type, generic classes)?

Hard

What is the Method Resolution Order (MRO)?

How does attribute lookup work on a Python object?