Question 1

What are common useful string methods?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — no prior knowledge assumed beyond what a variable is.
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11 and on the page runner (Pyodide 0.25.1 = CPython 3.11.3). `removeprefix`/`removesuffix` need 3.9 or newer.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` — the precise word an interviewer expects you to use. Every output shown was produced by actua

Question 2

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 3

How does slicing with start:stop:step work?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — you only need to know what a list and a string are.
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11 and the page runner (Pyodide 0.25.1 = CPython 3.11.3); the bytecode comparison was also run on CPython 3.12.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` — the precise word an interviewer expects you to use. Every output shown was produced by actua

Question 4

What is the difference between append, extend and insert on a list?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — you need to know what a list is; nothing more.
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11 (64-bit Linux) and the page runner (Pyodide 0.25.1 = CPython 3.11.3, a 32-bit WebAssembly build).

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` — the precise word an interviewer expects you to use. Every output shown was produced by actually running the

Question 5

Is Python pass-by-value or pass-by-reference?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — especially if you learned C, C++ or Java first and want to map the terms.
**Difficulty:** Easy
**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 th

Question 6

How do pyproject.toml, tomllib, and uv/pyproject workflows fit together?

Accepted Answer

**Target Audience:** Beginners who have run `pip install` but never set up a project from scratch; also experienced developers who want the precise names of each part.
**Difficulty:** Easy
**Python version:** The `tomllib` examples were run on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3). The uv commands were run with uv 0.11.32 against CPython 3.12.3.

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

Question 7

How do you read and write files in Python?

Accepted Answer

**Target Audience:** Beginners learning Python, and anyone who wants the precise answer an interviewer expects
**Difficulty:** Easy
**Python version:** Behaviour checked on CPython 3.11 (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 of the examples on this pa

Question 8

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 9

What is the difference between shallow and deep copy?

Accepted Answer

**Target Audience:** Beginners who have been surprised by "I changed the copy and the original changed too", and anyone preparing for the classic interview follow-ups on `copy`.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). `copy.replace()` needs 3.13.

> **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

Question 10

What is the difference between an iterable and an iterator?

Accepted Answer

**Target Audience:** Python developers who loop over things every day and want the precise model behind it
**Difficulty:** Medium
**Python version:** Checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); the protocol is the same on 3.12 and 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 expects. Every output block in the **Verified** section wa

PYTHONDECODED.

Easy

What are common useful string methods?

What are mutable and immutable types in Python?

How does slicing with start:stop:step work?

What is the difference between append, extend and insert on a list?

Is Python pass-by-value or pass-by-reference?

How do pyproject.toml, tomllib, and uv/pyproject workflows fit together?

How do you read and write files in Python?

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

Medium

What is the difference between shallow and deep copy?

What is the difference between an iterable and an iterator?

What are dunder (magic) methods?

What do the global and nonlocal keywords do?

How does collections.deque work internally?

What is the @property decorator?

What is composition versus inheritance?

Polars vs pandas: when should you switch in 2025-26?

Hard

What is a metaclass?

How does the asyncio event loop schedule coroutines?

What is __init_subclass__ in Python?