[Tutor] Object Oriented References? and a couple of question s <--LONG

alan.gauld@bt.com alan.gauld@bt.com
Sat, 11 Aug 2001 19:25:56 +0100

> I'm trying to hack my way into the verdant valley that is 
> Pythonic Object Orientism.   
> Would anybody have any recommendations for info on the subject?

As usual i'll recommend my tutor :-)

Look for the OO topic then also read the OO section of 
the case study.
> I've got a Cook who will scan a directory for ingredients, sift them
> so that we are using only the ingredients we want, and then cook up a
> bunch of Python Objects.  Then I've got a Host who will deal with the
> Diners and handle all of their requests and make sure 
> everyone gets what
> they need.  I've also got a number of Recipies (object templates) for
> The Cook to use.  This is how I'm thinking I'll lay things out, let me
> know if this isn't the best OO way to do things.

Thats a problem statement. Remember that in general "people" 
make bad objects. So try to avoid the Cook, Host and Diners 
as objects at least initially. They may have a role to play as "control"
objects later.

That leaves: ingredients, requests, recipies.

> The Cook will first scan a directory, and make a list of all 
> the files. This will be his Pantry.

Oh another object - the pantry.

> He will then Sift through the pantry for the files required 
> by the Recipe.

Better to get the pantry to do its own sifting - it owns 
the ingredients, it is responsible for operating on them 
- a key principle of OOD is he who owns the data does 
the work... Sometimes called "Responsibility Driven Design".

> that will be refered to as the Ingredients.  These Ingredients will be
> used to Bake a bunch of objects that I plan to use later.

Which objects? Meals maybe? Each meal knows how to bake itself maybe?

> When I'm setting up the Cook class I'm thinking the gather(), sift(),
> and bake() will be methods of the class, 

No these are methods of the other classes that the cook controls by calling
on them.


cook = Cook()
pantry = Pantry()

And the makeMeal method might look like this:

class Cook:
   def makeMeal(meal)
      ingredients = pantry.gather()
      for item in ingredients
      return meal.bake(ingredients)

> I'm really foggy on how to properly use classes to create objects and
> then use those objects.  As you can see, I'm in dire need of 
> a good explanation.

Try Timothy Budd's OO Programming. He uses 5 languages to 
explain the key principles of OO - showing how each language 
has its own foibles but the underlying principles are the same...


Alan G