Question 1

How do you work with dictionaries (get, setdefault, items, merging)?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — no prior knowledge assumed beyond variables and lists.
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11 and the page runner (Pyodide 0.25.1 = CPython 3.11.3). The `|` and `|=` operators need Python 3.9 or newer.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` — the precise word an interviewer expects you to use. Every output shown was produced by act

Question 2

How does slicing with start:stop:step work?

Accepted Answer

**Target Audience:** Beginners preparing for Python interviews — you only need to know what a list and a string are.
**Difficulty:** Easy
**Python version:** Checked on CPython 3.11 and the page runner (Pyodide 0.25.1 = CPython 3.11.3); the bytecode comparison was also run on CPython 3.12.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` — the precise word an interviewer expects you to use. Every output shown was produced by actua

Question 3

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 4

How are Python dictionaries implemented and ordered?

Accepted Answer

**Target Audience:** Beginners who use dicts daily and want to know what happens underneath; anyone preparing for a Python interview.
**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 in the **Verified** section

Question 5

How does exception handling work in Python?

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 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

What is the @property decorator?

Accepted Answer

**Target Audience:** Python beginners who know classes, up to candidates expected to explain descriptors
**Difficulty:** Medium
**Python version:** behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime); error messages also checked on 3.12 and 3.13

> **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

Question 8

Why is using a mutable default argument dangerous?

Accepted Answer

**Target Audience:** Beginners to intermediate — you can write functions with default values.
**Difficulty:** Medium
**Python version:** Behaviour checked on CPython 3.11.17 and in the page runner (Pyodide 0.25.1, CPython 3.11.3); the dataclass error message was also checked on 3.12.3.

> **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 actual

Question 9

What is composition versus inheritance?

Accepted Answer

**Target Audience:** Developers who can write classes and subclasses and want to know how to choose between the two main ways of reusing code.
**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

Question 10

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

PYTHONDECODED.

Easy

How do you work with dictionaries (get, setdefault, items, merging)?

How does slicing with start:stop:step work?

What does if name == "main" do?

How are Python dictionaries implemented and ordered?

How does exception handling work in Python?

Medium

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

What is the @property decorator?

Why is using a mutable default argument dangerous?

What is composition versus inheritance?

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

How does heapq work in Python?

How does a + b dispatch in Python?

How does yield from delegation work?

Hard

How does Python manage memory and garbage collection?

How do TypeAdapter, strict mode, and computed fields work in Pydantic v2?

How does the asyncio event loop schedule coroutines?

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?

PYTHONDECODED.

Easy

How do you work with dictionaries (get, setdefault, items, merging)?

How does slicing with start:stop:step work?

What does if __name__ == "__main__" do?

How are Python dictionaries implemented and ordered?

How does exception handling work in Python?

Medium

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

What is the @property decorator?

Why is using a mutable default argument dangerous?

What is composition versus inheritance?

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

How does heapq work in Python?

How does a + b dispatch in Python?

How does yield from delegation work?

Hard

How does Python manage memory and garbage collection?

How do TypeAdapter, strict mode, and computed fields work in Pydantic v2?

How does the asyncio event loop schedule coroutines?

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?

What does if name == "main" do?

What is the difference between getattr and getattribute?

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