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 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

What is uv and why is it replacing pip/poetry in 2025-26?

Accepted Answer

**Target Audience:** Python developers who know pip and venv and keep hearing about uv, and candidates asked about modern Python tooling
**Difficulty:** Easy
**Python version:** The in-page examples were run on CPython 3.11 and in this page (Pyodide 0.25.1, CPython 3.11.3). The shell sessions were run with uv 0.11.32 on Linux against python3.11; uv changes quickly, so check `uv --version` and the docs for your release.

> **How to read this doc:** Each idea is explained in plain language first,

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 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

Question 6

How does Pydantic v2 validation work (BaseModel, field_validator, mode)?

Accepted Answer

**Target Audience:** Python developers who have seen Pydantic models in FastAPI or config code and want to know exactly what runs, in which order, when data is validated.
**Difficulty:** Medium
**Python version:** Pydantic examples verified with pydantic 2.13.5 (pydantic-core 2.46.5) on CPython 3.13. Pydantic is a third-party package, so those two examples are shown as code for the playground; the first example is a standard-library model of the pipeline that runs in the page on Pyodide 0.25.1 (

Question 7

How does Polars LazyFrame query optimization work?

Accepted Answer

**Target Audience:** Developers who have used Polars (or pandas) and want to explain what the lazy API does under the hood.
**Difficulty:** Medium
**Python version:** The two examples are standard-library toy engines that run in the page (Pyodide 0.25.1, CPython 3.11.3) and were checked on CPython 3.11. **Polars itself was not installed in our verification environment**, so Polars snippets here were not run; Polars behaviour is described from the Polars user guide and API reference, cited inline

Question 8

What is the LEGB rule for scope resolution?

Accepted Answer

**Target Audience:** Python beginners and intermediate developers preparing for interviews — you only need to know what a function is.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13; the examples run 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, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output shown was produced by

Question 9

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 10

What is the difference between classmethod and staticmethod?

Accepted Answer

**Target Audience:** Beginners who know what `self` is, up to intermediate developers preparing for Python interviews.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11 and 3.13 (identical output except where a version is named); every in-page example also runs 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:

PYTHONDECODED.

Easy

Why use virtual environments?

What does if name == "main" do?

What is uv and why is it replacing pip/poetry in 2025-26?

What is the difference between repr and str?

Medium

What is the difference between an iterable and an iterator?

How does Pydantic v2 validation work (BaseModel, field_validator, mode)?

How does Polars LazyFrame query optimization work?

What is the LEGB rule for scope resolution?

What is the difference between new and init?

What is the difference between classmethod and staticmethod?

What are bound and unbound methods in Python?

What is a deadlock in Python?

Hard

What is a weak reference in Python?

What is the Method Resolution Order (MRO)?

What is the difference between getattr and getattribute?

How does attribute lookup work on a Python object?

What is the descriptor protocol?

What is the difference between del, del, and garbage collection?

How do Unpack and TypedDict enable type-safe **kwargs?

PYTHONDECODED.

Easy

Why use virtual environments?

What does if __name__ == "__main__" do?

What is uv and why is it replacing pip/poetry in 2025-26?

What is the difference between __repr__ and __str__?

Medium

What is the difference between an iterable and an iterator?

How does Pydantic v2 validation work (BaseModel, field_validator, mode)?

How does Polars LazyFrame query optimization work?

What is the LEGB rule for scope resolution?

What is the difference between __new__ and __init__?

What is the difference between classmethod and staticmethod?

What are bound and unbound methods in Python?

What is a deadlock in Python?

Hard

What is a weak reference in Python?

What is the Method Resolution Order (MRO)?

What is the difference between __getattr__ and __getattribute__?

How does attribute lookup work on a Python object?

What is the descriptor protocol?

What is the difference between del, __del__, and garbage collection?

How do Unpack and TypedDict enable type-safe **kwargs?

What does if name == "main" do?

What is the difference between repr and str?

What is the difference between new and init?

What is the difference between getattr and getattribute?

What is the difference between del, del, and garbage collection?