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 difference between a module and a package?

Accepted Answer

**Target Audience:** Beginners learning Python, and anyone preparing for a Python interview
**Difficulty:** Easy
**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)

> **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 copied from a real run of the examples on th

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

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 5

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 6

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 7

How do generator pipelines stream data lazily?

Accepted Answer

**Target Audience:** Developers who know basic generators and want to process big or endless data streams; candidates preparing for "how would you process a huge file?" questions.
**Difficulty:** Medium
**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:** Each idea is explained in plain words first, then tagged with `📌 Interview term:` — that is the exact vocabulary an interviewer expects to hear. Every ou

Question 8

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

Accepted Answer

**Target Audience:** Python developers who use pandas and are deciding whether a project should move to Polars, and candidates asked "pandas or Polars?" in a data-engineering interview.
**Difficulty:** Medium
**Python version:** The pandas example ran on CPython 3.13 with **pandas 3.0.5**. The toy model runs in the page (Pyodide 0.25.1, CPython 3.11.3). **Polars was not installed in our verification environment**, so the Polars snippets on this page were not executed: they are shown for the API

Question 9

What are lambda functions and map/filter/reduce?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — you know lists, loops and basic functions.
**Difficulty:** Medium
**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 the code; the **Verified** sect

Question 10

What is a deadlock in Python?

Accepted Answer

**Target Audience:** Developers who use locks or threads and want to understand why programs freeze — no prior concurrency knowledge needed beyond what a lock is.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11, 3.12 and 3.13; the in-page example runs 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 outpu

PYTHONDECODED.

Easy

Why use virtual environments?

What is the difference between a module and a package?

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

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

Medium

How does Polars LazyFrame query optimization work?

How do you run external commands with subprocess safely?

How do generator pipelines stream data lazily?

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

What are lambda functions and map/filter/reduce?

What is a deadlock in Python?

What are generators and why use them?

What is the itertools module used for?

Hard

How does CPython execute your code?

How does multiprocessing share data between processes?