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 TOML with tomllib in Python 3.11+?

Accepted Answer

**Target Audience:** Python developers who configure apps or tools, explained from zero
**Difficulty:** Easy
**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 of the examples on this page.

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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

How do you convert between Python objects and JSON?

Accepted Answer

**Target Audience:** Beginners learning Python, and developers who want the edge cases an interviewer probes
**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 page

Question 6

What values are falsy 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); one version-dependent case was checked on 3.12 and 3.13 as noted.

> **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 out

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

What is the difference between __new__ and __init__?

Accepted Answer

**Target Audience:** Python developers who write classes with `__init__` and have seen `__new__` in library code or an interview question.
**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).

> **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 a

Question 9

How do you write tests in Python with unittest and pytest?

Accepted Answer

**Target Audience:** Beginners writing their first tests, and developers who use one framework and need to talk confidently about both.
**Difficulty:** Medium
**Python version:** The unittest and mock examples were run on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, CPython 3.11.3). The pytest runs were done with pytest 9.1.1 on CPython 3.13, because pytest is not part of the standard library and cannot run in the page.

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

Question 10

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

PYTHONDECODED.

Easy

What are mutable and immutable types in Python?

How do you read TOML with tomllib in Python 3.11+?

How does the in operator work in Python?

What are Python's built-in data types?

How do you convert between Python objects and JSON?

What values are falsy in Python?

Medium

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

What is the difference between new and init?

How do you write tests in Python with unittest and pytest?

What are generics and TypeVar in Python?

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

Hard

What is TypeVarTuple and Unpack for variadic generics?