Question 1

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 2

What is a race condition and how do locks prevent it?

Accepted Answer

**Target Audience:** Developers starting with threads who want to understand why shared data goes wrong — no prior concurrency knowledge needed.
**Difficulty:** Medium
**Python version:** Bytecode and behaviour checked on CPython 3.11, 3.12 and 3.13 (standard GIL builds); 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 exp

Question 3

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 4

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 5

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 6

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 7

How does exception propagation work up the call stack?

Accepted Answer

**Target Audience:** Python developers who can write try/except and want to know exactly what happens between a raise and its handler
**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 thread and process example on CPython 3.12

> **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 bloc

Question 8

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

Question 9

What is a context manager?

Accepted Answer

**Target Audience:** Python learners who use `with open(...)` without knowing what it does, up to candidates who need the exact protocol and CPython details.
**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); spot checks on 3.12 and 3.13 are noted where made.

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

Question 10

What is a generator's lifecycle?

Accepted Answer

**Target Audience:** Python developers who use generators and want to know exactly what state one is in, and what happens to its clean-up code
**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 version-specific example on CPython 3.13, with 3.12 compared

> **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

PYTHONDECODED.

Medium

What is the difference between shallow and deep copy?

What is a race condition and how do locks prevent it?

What is the Global Interpreter Lock (GIL)?

How does collections.deque work internally?

How do you run external commands with subprocess safely?

What is the LEGB rule for scope resolution?

How does exception propagation work up the call stack?

What is a deadlock in Python?

Hard

How does multiprocessing share data between processes?

How does async/await work in Python?

What is Python 3.12's free-threaded (no-GIL) build and what changes?