Creature Lab — Classes & objects

How to play

Each level gives you a specification: expressions on the left, the value each should produce on the right. Write the class, press Run, and every row is ticked individually — so a class that works for one creature and quietly shares state between two tells you exactly which case broke.

What each level teaches

1. The blueprint — class, __init__ and self.
2. Teach it a trick — methods, and how they reach their own data.
3. Take a hit — changing an object's state rather than returning a new one.
4. Make it printable — __str__, instead of <__main__.Creature object at 0x7f…>.
5. Count the specimens — a class attribute, shared by every instance.
6. Are they the same? — __eq__, and what == means by default.

What self actually is

It is not a keyword and there is nothing magic about the name. When you call sparky.describe(), Python calls Creature.describe(sparky) — the object is passed in as the first argument, and self is just the conventional name for it. That is why every method needs it in the signature, and why forgetting it gives you "takes 0 positional arguments but 1 was given", which is one of the most confusing error messages a beginner meets.

The trap in level 5

Level 1 already checks that two creatures have independent hp, and level 5 is the mirror image:

• self.count = … creates an attribute on that one object. Every creature gets its own, so the total is always 1.
• Creature.count = … changes the attribute on the class, which every instance shares.

Reading is the confusing part: self.count reads the class attribute perfectly well if no instance attribute exists. So the count appears to work until something writes to it through self, and from that moment the two silently diverge.

Dunder methods are hooks, not decoration

__str__ and __eq__ are not special-cased by the language — they are how Python asks your object a question. print(x) calls __str__, x == y calls __eq__, len(x) calls __len__, for i in x calls __iter__. Implement the hook and your class works with the ordinary syntax everyone already knows. The written version, including properties and dataclasses, is in the tutorial on Python classes and objects.

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Frequently asked questions

Keep going

• Read the classes & objects tutorial — the written explanation behind every level here.
• Variable Vault — variables & data types.
• Operator Arcade — numbers & operators.
• String Surgeon — strings & slicing.
• 20 Python practice exercises with worked solutions.

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