Week 02 · Classes and Objects

Mission Quest

Mission Quest: Blueprint cards

// A class is a blueprint, and each object keeps its own state

⏱ about 30 min

Saturday: Mission Quest: Blueprint Cards

Comet has a pile of index cards and two colored pens. On the top card she writes Sample, then lists label, kind and grams.

"That is the class card," she says. "Now I fill one object card for each real sample, with real values."

Wren holds up a lamp from the shelf. "What does the evidence say?" he asks. "Could a lamp have a class card too? It has parts and it does things."

Nova casts a soft glow on the lamp. "How can I help?" she asks. "List what every lamp has, then what every lamp can do."

Comet passes you a blank card. "Find a family member, lead programmer. Pick something at home."

Class cards and object cards

An object is one instance of a class. The class is the blueprint for the attributes and behaviors every object of that type has.

A class card lists the attributes and behaviors. An object card gives one object's own values for each attribute.

Card	Attributes	Behaviors
Class card: Lamp	color, brightness, is_on	turn_on, turn_off, describe
Object card: desk lamp	color = green, brightness = 3, is_on = False	(same as the class)
Object card: hall lamp	color = white, brightness = 5, is_on = True	(same as the class)

◇ BLUEPRINT AND INSTANCE CARDS

You need index cards or scraps of paper and a pen. Choose one kind of household object together, such as a chair, a mug or a backpack.
Write one class card: the class name, three or four attributes, and two or three behaviors.
Each of you fills one object card for a real one in your home, with its own value for every attribute.
Compare your object cards. Which attributes have the same value? Which are different?
Ask each other: would two identical mugs need two object cards? Why?

For a grown-up

The idea today is the difference between a blueprint, the class, and the things made from it, the objects. Everyday objects make it concrete.
Talk about it: what could go wrong if every object card could use different attributes, the way Comet's dicts could?

CHECK YOUR CARDS

Read the question.
Tap your answer.

Which card is the blueprint?

Two lamps share a class card. Do they share brightness values?

In Python, which method sets an object's starting values?

Copy your class card here. Which attribute was hardest to agree on?Write the first line of a Python class for your object, and the def line of its __init__.

Great teamwork. Next week Pip gets a class of its own, and the crew learns to guard its state.

Grown-up checked my answers

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