[New Concept Docs]: Class Composition in Python

[BethanyG]

This issue describes how to implement the class-composition concept docs. You can find the related concept exercise issue here

If you have not yet contributed to concept documents, this issue will require some upfront reading to give you the needed background knowledge.

✅ Getting started

Please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time.

General Contributing Docs:

• Contributing to Exercism | Exercism and GitHub | - Contributor Pull Request Guide
• What are those Weird Task Tags about?
• Exercism Formatting and Style Guide
• Exercism Markdown Specification
• Reputation

Documents on Language Tracks and Concepts:

• Building Language Tracks: An Overview
• What are Concepts?
• Concept Specifications

🎯 Goal

These concept docs are meant to teach an understanding/use of class-composition in Python.

💡 Learning objectives

• more fully understand the uses (and possible abuses) of classes in Python
• understand what composition is vs inheritance and multiple-inheritance in classes
• learn a bit about the pros and cons of using composition vs inheritance or mixins in classes
• learn about the "Composition over inheritance" principle
• learn a bit about dependency injection / inversion of control as a composition design pattern

🚫 Out of scope

Concepts and Subjects that are Out of Scope (click to expand)

• metaclasses
• dataclasses
• decorators outside of @property, @staticmethod, and @classmethod (the student should already be exposed to these in the prerequisite exercise)
• generators
• coroutines
• descriptors (these will get their own exercise)
• using a class as a decorator
• performance considerations

🤔 Concepts

Concepts & Related Concepts Under This Topic (click to expand)

• classes
• composition in OOP
• encapsulation in OOP
• inheritance in OOP
• dependency injection

↩️ Prerequisites

These are the concepts/concept exercises the student should be familiar with before taking on/learning this concept.

Prereqs (click to expand)

• basics
• booleans
• classes
• class-customization
• class-inheritance
• comparisons
• decorators
• dicts
• functions
• higher-order functions
• iteration
• lists
• numbers
• sequences
• sets
• strings
• tuples

📚 Resources to refer to

Resources (click to expand)

• classes (Python tutorial)
• Real Python: Object-Oriented Programming in Python 3
• Real Python: Inheritance and Composition: A Python OOP Guide
• What is Composition (OOP in Python)
• What is Composition -- good explanation, but it uses Java
• Inheritance, Composition, and Aggregation in Python
• The Composition over Inheritance Principle
• Pythonic Dependency Injection: A Practical Guide
• Method Delegation in Python
• Python Data Model - Python Docs
• Stack Overflow: Composition and Aggregation in Python

📁 Files to Be Created

File Detail for this Exercise

Please see the following for more details on these files: concepts links.json For more information, see concept links file The same resources listed in this issue can be used as a starting point for the concepts/links.json file, if it doesn't already exist. If there are particularly good/interesting information sources for this concept that extend or supplement the concept exercise material & the resources already listed -- please add them to the links.json document. Concept about.md For more information, see Concept about.md This file provides information about this concept for a student who has completed the corresponding concept exercise. It is intended as a reference for continued learning. Concept introduction.md For more information, see Concept introduction.md This can also be a summary/paraphrase of the about.md document listed above, and will provide a brief introduction of the concept for a student who has not yet completed the associated concept or practice exercises. It should contain a good summation of the concept, but not go into lots of detail. Concept .meta/config.json Entries For more information, see Concept .meta/config.json This file is likely already stubbed out. Remember to add a concept blurb of less than 350 characters. Please also add your GitHub username to the "authors" array, and any contributor GitHub usernames to the "contributors" array.

🎶 Implementation Notes

• Example code should only use syntax & concepts introduced within these docs or one of the prerequisite concept exercises or documents. Where possible, please use REPL formatting, unless you are demonstrating pseudo code or a long code block. Please do not use syntax not previously covered in prerequisite topics or exercises. Please also follow PEP8 guidelines.
• Our markdown and JSON files are checked against prettier . We recommend setting prettier up locally and running it prior to submitting your PR to avoid any CI errors.

🆘 Next Steps & Getting Help

1. If you'd like to work on this issue, comment saying "I'd like to work on this" (there is no real need to wait for a response, just go ahead, we'll assign you and put a [claimed] label on the issue).
2. If you have any questions while implementing, please post the questions as comments in here, or contact one of the maintainers on our Slack channel.