Write a blueprint, make a real one

01 / 07

Somewhere to write down what a member is

What hurt in the last lesson was how free a dictionary is. Leave an item out and nobody complains, and nowhere says what a member is.

The machinery for declaring that in one place is the class. Think of it as writing a single blueprint.

02 / 07

Writing class

Put a name after class and close the line with a :. From the next line, indent by four spaces and write the inside. The same shape as if and def.

There is nothing to put inside yet, and Python will not let the inside be empty. Put down one line of pass, which says "do nothing", and move on.

class Member:    passprint("blueprint made")

Result

blueprint made

03 / 07

Names start with a capital

The Python convention is for a class name to start with a capital letter. That way you can tell member, the container variable, from Member, the blueprint, at a glance.

Make the name what you would call one of them. Even a blueprint for handling a crowd of members is Member, not Members.

class Member:    passclass Product:    passprint("two blueprints")

Result

two blueprints

04 / 07

Making a real one from the blueprint

Stare at a cake tin all you like and no cake appears. Writing the blueprint on its own brings not one scrap of real data into being.

To make a real one, call the class name with () after it. What comes out is called an instance.

class Member:    passmember = Member()member.name = "Aoi"print(member.name)

Result

Aoi

05 / 07

Values go in and out through a dot

A value an instance holds is called an attribute. Reading and writing both use the shape instance.name.

What was member["name"] with a dictionary is member.name with a class. No square brackets and no quotes, which reads more easily.

class Member:    passmember = Member()member.name = "Aoi"member.years = 3print(member.name)print(member.years)

Result

Aoi
3

06 / 07

As many as you like from one blueprint

Instances made from the same class are separate containers. Change an attribute on one and the other stays as it was.

One cake tin, and the cakes that come out of it are each their own.

class Member:    passa = Member()a.name = "Aoi"b = Member()b.name = "Hinata"print(a.name)print(b.name)

Result

Aoi
Hinata

07 / 07

You are still putting it in by hand

At the moment you make one with Member() and then write .name = ... yourself. Five items means five lines, and ten members means fifty.

And nothing stops you when you forget one, so you are no better off than you were with dictionaries. The machinery for having the contents ready as it is made turns up next lesson.