Question 1

What are Python's built-in data types?

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 is duck typing in Python?

Accepted Answer

**Target Audience:** Beginners who know functions and classes, and anyone who wants a crisp interview definition with real examples.
**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 in the **Verified** section w

Question 3

What are abstract base classes and Protocols?

Accepted Answer

**Target Audience:** Developers who know classes and inheritance and have seen `ABC` or `Protocol` in a codebase, up to candidates who need to compare nominal and structural typing precisely.
**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 differences we saw on 3.12 and 3.13 are called out.

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

Question 4

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 5

What are generics and TypeVar in Python?

Accepted Answer

**Target Audience:** Python developers who use type hints and want to write functions and classes that work for many types without falling back to `Any`.
**Difficulty:** Medium
**Python version:** Runtime behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3). Type-checker output is from mypy 2.3.1 with `--python-version 3.11`. This page uses the `TypeVar` spelling that works on every supported version; the 3.12 shorthand is covered in [Python 3.12 type para

Question 6

How do you enforce match/case exhaustiveness in Python?

Accepted Answer

**Target Audience:** Python developers who use `match` or `if`/`elif` chains over enums or tagged types and want missing cases caught automatically.
**Difficulty:** Medium
**Python version:** `assert_never` needs Python 3.11+ (or `typing_extensions`). Runtime behaviour checked on CPython 3.11; both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3). Checker output is from mypy 2.3.1 and pyright 1.1.414.

> **How to read this doc:** Ideas come in plain language first. Terms an inter

Question 7

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 8

What does typing.Self do and why does it beat TypeVar for methods?

Accepted Answer

**Target Audience:** Python developers who write class hierarchies with chainable methods or alternative constructors and use a type checker.
**Difficulty:** Medium
**Python version:** `typing.Self` exists from Python 3.11. Runtime behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3). Type-checker output is from mypy 2.3.1 with `--python-version 3.11`.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interv

Question 9

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

Question 10

How do type hints work in Python?

Accepted Answer

**Target Audience:** Python developers who have seen `def f(x: int) -> str:` and want to know what it really does, from the interpreter and from a type checker.
**Difficulty:** Medium
**Python version:** Runtime behaviour checked on CPython 3.11.17 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). Type-checker output is from mypy 2.3.1 (with `--python-version 3.11`) and pyright 1.1.414.

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

PYTHONDECODED.

Easy

What are Python's built-in data types?

What is duck typing in Python?

Medium

What are abstract base classes and Protocols?

What is the difference between class variables and instance variables?

What are generics and TypeVar in Python?

How do you enforce match/case exhaustiveness in Python?

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

What does typing.Self do and why does it beat TypeVar for methods?

What is EAFP vs LBYL in Python?

How do type hints work in Python?

Hard

How does typing.Never enable exhaustive match checking?

What is TypeVarTuple and Unpack for variadic generics?

How do Unpack and TypedDict enable type-safe **kwargs?