Question 1

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 2

What is the difference between shallow and deep copy?

Accepted Answer

**Target Audience:** Beginners who have been surprised by "I changed the copy and the original changed too", and anyone preparing for the classic interview follow-ups on `copy`.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). `copy.replace()` needs 3.13.

> **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

Question 3

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 4

What is the Global Interpreter Lock (GIL)?

Accepted Answer

**Target Audience:** Python developers who have used or heard of threads and want to understand what CPython actually allows. No operating-systems background is assumed.
**Difficulty:** Medium
**Python version:** Behaviour checked on standard (GIL) builds of CPython 3.11, 3.12 and 3.13. Example 1 also runs in the page on Pyodide 0.25.1 (CPython 3.11.3); Example 2 uses real threads, which the browser build cannot start, so it is verified on 3.12.

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

Question 5

How does collections.deque work internally?

Accepted Answer

**Target Audience:** Developers who use `deque` as "the fast queue" and want to explain why, up to candidates expected to describe the CPython data structure.
**Difficulty:** Medium
**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); the block-growth numbers were identical on python3.13.

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

Question 6

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 7

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 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 is the difference between the call stack and the heap in Python?

Accepted Answer

**Target Audience:** Developers coming from C, C++, Java or Go who want to map "stack vs heap" onto Python, and Python developers debugging recursion or memory
**Difficulty:** Medium
**Python version:** Checked on CPython 3.11, 3.12 and 3.13 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); all gave identical output

> **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 expect

Question 10

How does functools.lru_cache work?

Accepted Answer

**Target Audience:** Python developers who know basic decorators, up to candidates asked about caching internals
**Difficulty:** Medium
**Python version:** behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime)

> **How to read this doc:** Every idea is first explained in plain words. When a term has a precise name an interviewer expects to hear, it is introduced as a 📌 **Interview term** callout. The Quick Glossary at the end lists all of them.

## 1. Why This Ev

PYTHONDECODED.

Easy

How do you read and write files in Python?

Medium

What is the difference between shallow and deep copy?

What is the difference between bytes and str?

What is the Global Interpreter Lock (GIL)?

How does collections.deque work internally?

How do generator pipelines stream data lazily?

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

How do you measure and profile Python performance?

What is the difference between the call stack and the heap in Python?

Hard

How does Python manage memory and garbage collection?

What is a weak reference in Python?

How does multiprocessing share data between processes?

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

PYTHONDECODED.

Easy

How do you read and write files in Python?

Medium

What is the difference between shallow and deep copy?

What is the difference between bytes and str?

What is the Global Interpreter Lock (GIL)?

How does collections.deque work internally?

How do generator pipelines stream data lazily?

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

How do you measure and profile Python performance?

What is the difference between the call stack and the heap in Python?

Hard

How does Python manage memory and garbage collection?

What is a weak reference in Python?

How does multiprocessing share data between processes?

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

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