Question 1

Why use virtual environments?

Accepted Answer

**Target Audience:** Beginners setting up their first Python projects, and anyone preparing for a Python interview
**Difficulty:** Easy
**Python version:** Example 1 was run on CPython 3.11 and in this page (Pyodide 0.25.1, CPython 3.11.3); Example 2 creates a real environment, so it was run on CPython 3.12 outside the browser. The shell output below is from python3.11 on Linux.

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

Question 2

What is the enum module and when should you use it?

Accepted Answer

**Target Audience:** Python beginners and intermediate developers who use strings or numbers as status codes and want to know what `enum` adds
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); differences in 3.12 are called out where we ran 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. Ever

Question 3

What are f-strings?

Accepted Answer

**Target Audience:** Beginners formatting their first output; candidates who want the format mini-language, the CPython detail and the version history.
**Difficulty:** Easy
**Python version:** f-strings exist since Python 3.6. Examples 1 and 2 checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); Example 3 needs 3.12+ and was checked on 3.12.3 and 3.13.16.

> **How to read this doc:** Every idea is explained in plain words first, then tagged `📌 Interview term:` wit

Question 4

What is the difference between __repr__ and __str__?

Accepted Answer

**Target Audience:** Beginners who have written a class or two, up to intermediate developers revising for interviews.
**Difficulty:** Easy
**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 term:` and defined right there. All

Question 5

What are dunder (magic) methods?

Accepted Answer

**Target Audience:** Beginners who know how to write a basic `class`, up to intermediate developers preparing for Python 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 term:` and defined

Question 6

Why is using a mutable default argument dangerous?

Accepted Answer

**Target Audience:** Beginners to intermediate — you can write functions with default values.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); the dataclass error message was also checked on 3.12.3.

> **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 actual

Question 7

Why does 0.1 + 0.2 != 0.3 in Python, and when do you use Decimal or Fraction?

Accepted Answer

**Target Audience:** Python developers who have been surprised by a float result and want to know exactly why, and which numeric type to use instead
**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

Question 8

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 9

What are dataclasses?

Accepted Answer

**Target Audience:** Python developers who write classes by hand and want to know what `@dataclass` generates, which options matter, and where its limits are.
**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). `slots`, `kw_only` and `KW_ONLY` need 3.10+.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact

Question 10

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

PYTHONDECODED.

Easy