Question 1

What is the difference between a module and a package?

Accepted Answer

**Target Audience:** Beginners learning Python, and anyone preparing for a Python interview
**Difficulty:** Easy
**Python version:** Every example was run 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 examples on th

Question 2

What does if __name__ == "__main__" do?

Accepted Answer

**Target Audience:** Beginners writing their first scripts and modules; anyone who has copied the line without knowing why.
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11.17, 3.12.3 and 3.13.16 (identical output) and in the page runner (Pyodide 0.25.1, CPython 3.11.3).

> **How to read this doc:** Ideas come in plain words first, then a `📌 Interview term:` callout with the precise name an interviewer uses. Every output below is copied from a real run; the **Verified** section

Question 3

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 4

What useful types does the collections module provide?

Accepted Answer

**Target Audience:** Python learners who know `list` and `dict` and want the tools that make everyday code shorter, up to candidates who need the gotchas and internals.
**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. Ever

Question 5

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 6

What is monkey patching in Python?

Accepted Answer

**Target Audience:** Developers who have seen `mock.patch` in tests or a "quick fix" that reassigns a library function, up to candidates who must explain scope, the import trap and the risks.
**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); spot checks on 3.12 and 3.13 are noted.

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

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

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

Question 9

How does the Python import system work?

Accepted Answer

**Target Audience:** Python developers who know how to import things and want to know how importing works, and candidates for senior Python interviews
**Difficulty:** Hard
**Python version:** Every example was run on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); Example 2 also gave identical output on 3.13

> **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

Question 10

How does CPython execute your code?

Accepted Answer

**Target Audience:** Python developers who have written functions and loops and want to know what the interpreter does with them. No C or compiler background is assumed.
**Difficulty:** Hard
**Python version:** Behaviour and `dis` output checked on CPython 3.11, with the differences on 3.12 and 3.13 shown where they matter. Both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

> **How to read this doc:** Every idea is explained in plain words first. When a word appears that an

PYTHONDECODED.

Easy

What is the difference between a module and a package?

What does if name == "main" do?

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

Medium

What useful types does the collections module provide?

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

What is monkey patching in Python?

What is pickling and what are its risks?

How do type hints work in Python?

Hard

How does the Python import system work?

How does CPython execute your code?

How does multiprocessing share data between processes?

What is __init_subclass__ in Python?

What is Python 3.12's free-threaded (no-GIL) build and what changes?