[workshop-lite] lesson 02: data objects

[Michal Wallace]

# lesson 02: data objects

* A Quick Intro to Objects

When I plan a new web app, I start with the data objects - objects
whose main purpose is to hold data.

What are objects? Instead of boring you with definitions, let's 
use an example.

To begin, fire up a python interpreter. If you don't have 
python yet, you can download it from http://www.python.org/

Since python's interpreter is interactive, you can follow
along with the example by typing them at the ">>>" prompt. 


* My Dog Spot

Let's create a simple class to represent dogs. By convention, 
we name classes with capital letters:

>>> class Dog: pass
...

The "..." means that python is waiting for more input.
We don't have any more input (that's what "pass" means)
so just press enter.

That's all it took to create a class called Dog. 

A Dog class should contain information that applies
to dogs. For example, most dogs have four legs:

>>> Dog.legs = 4

But suppose we want to talk about a particular dog - an
instance of the class Dog:

>>> spot = Dog()

Now we have a Dog named "spot" (we name instances and
other variables in lower case to tell them apart from
classes). Since spot is a Dog, he should have four legs.
To test that, we can ask python:

>>> spot.legs
4

Yep, spot has four legs.

But suppose spot has a horrible        skiing accident. (Don't worry,
this is staged - no animals were hurt in the filming of this
workshop.)

>>> spot.legs = 2
>>> Dog.legs
4
>>> spot.legs
2 

As you can see, python lets us record the fact that while most Dogs
have four legs, poor spot has only two.


* Spot is a Data Object

We said that objects are reusable chunks of data and behavior.  The
Dog class has a little bit of data (legs=4), but no real behavior. All
you can do is instantiate it (make an instance of it).

Since a Dog instance's main purpose is to store data, spot is a data
object. If we wanted, we could record all kinds of data about spot, 
just by adding more fields to the Dog class: name, breed, color, and
birthday might all make sense.


* Who Let the Dogs Out?

Let's say we're building a web app for the neighborhood kennel.
We're going to use a Dog object to represent each dog. Of course,
we can't change our program every time a dog checks in or out. 
We'd go nuts! Instead, we'll store our data objects in some kind
of database.

In an ideal world, we'd store objects in an object database.
There is at least one  object database for python, but I've
never used it, so I can't say much about it. ( http://www.amk.ca/zodb/ )

I will say that, in general, object databases are hard to query.
Asking for dog # 5 is quick and painless, but asking "how many beagles
are in the kennel?" usually requires actual code to loop through and
count, and I don't like that.

(I hope I'm not spreading FUD here - if I am, please let me know,
because I would *love* to work with a really useful object 
database. LDAP servers seem to have a bit more strength in the
object-querying department, but I don't have much experience 
with LDAP either.)

Meanwhile, I use MySQL. MySQL doesn't know anything about objects, but
it's fairly easy to write some generic routines to convert between
MySQL records and python objects. Plus, MySQL is very fast, and gives
us access to SQL (a powerful and somewhat standardized query
language). 

We'll take a closer look at how this object-relational mapping is done
in the next lesson.

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