Question 1

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 2

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 3

How does the in operator work in Python?

Accepted Answer

**Target Audience:** Beginners to intermediate Python developers — you should know lists, dicts and sets, and roughly what a class is.
**Difficulty:** Easy
**Python version:** Behaviour, error messages and bytecode 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:** Ideas come in plain language first. Words an interviewer expects you to use are tagged `📌 Interview term:` and defined on the spot; all of them are repeated in

Question 4

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 5

What is structural pattern matching (match/case)?

Accepted Answer

**Target Audience:** Python developers who know `if`/`elif`, lists, dicts and classes and have never used `match`, plus anyone who wants the exact rules an interviewer probes.
**Difficulty:** Medium
**Python version:** `match` needs Python 3.10+. Behaviour and bytecode checked on CPython 3.11 (gotchas re-run on 3.13); both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

> **How to read this doc:** Ideas come in plain language first. The words an interviewer expects you to use

Question 6

What is the difference between bytes and str?

Accepted Answer

**Target Audience:** Python developers who handle files, network data or APIs, explained from zero
**Difficulty:** Medium
**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 page.

## 1.

Question 7

How do you run external commands with subprocess safely?

Accepted Answer

**Target Audience:** Developers who have used `os.system` or copied a `subprocess` line from the internet, and want to know what each argument does and why it matters.
**Difficulty:** Medium
**Python version:** Example 1 (`shlex`) was run on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, CPython 3.11.3). Examples 2 and 3 start real processes, which a browser runtime cannot do, so they are shown with an "Open in playground" button and were run on python3.12 (identical output on

Question 8

What are common Python security pitfalls?

Accepted Answer

**Target Audience:** Developers who write Python services or scripts and want a concrete checklist; no security background assumed.
**Difficulty:** Medium
**Python version:** Examples 1 and 2 were run on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, CPython 3.11.3). Example 3 needs `sqlite3` and the `tarfile` filters, which the page runtime does not have, so it was run on python3.12 (and gave identical output on python3.13).

> **How to read this doc:** Each idea is explained

Question 9

Why does Python cache small integers?

Accepted Answer

**Target Audience:** Python developers who know what a variable is and have seen `is` and `==`. No knowledge of CPython's C code is needed.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11 (64-bit), with 3.12 and 3.13 where they differ. Examples 1 and 2 also run in the page on Pyodide 0.25.1 (CPython 3.11.3); Example 3 needs 3.12+.

> **How to read this doc:** Each idea is explained in plain words first. When a word appears that an interviewer expects you to know, it

Question 10

What is the memory difference between a list comprehension and a generator expression?

Accepted Answer

**Target Audience:** Python developers who know both syntaxes and want to be able to quantify and explain the difference
**Difficulty:** Medium
**Python version:** Checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); bytecode compared with CPython 3.12. Byte counts are from 64-bit CPython; Pyodide is a 32-bit build, so the examples print comparisons, not raw numbers

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

PYTHONDECODED.

Easy

What are mutable and immutable types in Python?

How do you read and write files in Python?

How does the in operator work in Python?

What are Python's built-in data types?

Medium

What is structural pattern matching (match/case)?

What is the difference between bytes and str?

How do you run external commands with subprocess safely?

What are common Python security pitfalls?

Why does Python cache small integers?

What is the memory difference between a list comprehension and a generator expression?

What are generics and TypeVar in Python?

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

Hard

How does Python manage memory and garbage collection?

How does multiprocessing share data between processes?