Objects, classes, and composition over inheritance
By the end of today you can decide whether something should be a class or stay a function, explain what self actually is, and say why deep inheritance hierarchies cause more problems than they solve.
YesterdayOn Day 4 you learned that a function is a boundary. A class is a boundary around data and the operations that belong with it.
TomorrowTomorrow you meet the first structures worth building as classes, and the call stack becomes something you can see.
Why this matters
Almost every codebase you read will be organised around classes. Knowing when they earn their place, and when a plain function was enough, is the difference between structure and ceremony.
- Classes and objects
- Encapsulation
- Composition over inheritance
- When not to use a class
Learn it
75 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
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Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
4 hand-picked resources
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Build it
45 minWrite a shopping basket first as loose functions passing a dict around, then as a class. Add __repr__ and __eq__ to the class version. Then take one piece of behaviour that varies, such as a discount rule, and implement it once with inheritance and once with composition, and write two sentences on which you would keep.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
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Rate it
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Mastery tracking
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Recap
- 01self is the instance, passed as the first argument
- 02A class earns its place when several operations share state
- 03Deep inheritance scatters behaviour; composition keeps it explicit
- 04__repr__ pays for itself the first time you debug
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.