What is composition versus inheritance? | Interview Question

Q: What is composition versus inheritance?

**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 by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First A car manufacturer could design a new model in two ways. One: "the new model **is** the old model, with changes". Every part of the old car comes along, wanted or not, and if engineering changes the old car's chassis, the new one changes too. Two: "the new model **has** an engine, **has** a gearbox, **has** wheels". You pick each part, you can bolt in an electric engine next ye

The problem

Question presented to candidate: "People say 'favour composition over inheritance'. What is the difference between the two, can you show it in Python, and when is inheritance still the right choice?"

What a strong answer should cover:

Inheritance is an is-a relationship: the subclass gets every attribute and method of the base and can be used wherever the base is expected (substitutability)
Composition is a has-a relationship: the object holds other objects as attributes and calls them, exposing only its own interface
Why composition is often preferred: looser coupling, no fragile base class, parts can be swapped at runtime and replaced by fakes in tests, no subclass explosion when features combine
A concrete Python trap: subclassing dict and overriding __setitem__ is bypassed by dict.__init__ and update(); collections.UserDict (which wraps a dict) fixes it
When inheritance fits: a real is-a relationship, framework base classes and ABCs, small mixins; delegation with __getattr__ as a middle ground

Clarifying questions expected:

Is the new type genuinely a kind of the old one, usable everywhere the old one is?
Will behaviours need to be combined or swapped (formats, storage, transports)?

Code / implementation expected: Optional — the same feature built both ways, for example a Stack that inherits from list versus one that holds a list.

oop
composition

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

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A short nudge toward the approach. It will not give the answer away.

Solution

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 by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

A car manufacturer could design a new model in two ways. One: "the new model is the old model, with changes". Every part of the old car comes along, wanted or not, and if engineering changes the old car's chassis, the new one changes too. Two: "the new model has an engine

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11 min read · 12 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonIs-a versus has-a: a list-based Stack and the dict subclass trap

Practice

PythonComposing behaviour from parts, swapping them at runtime, and delegation

Practice

Discussion

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