[Tutor] Communication between classes

[Andrei]

Hi Greg,

Greg Perry wrote:
> I am still in the process of learning OOP concepts and 
 > reasons why classes should be used instead of functions etc.
> 
> One thing that is not apparent to me is the best way for 
 > classes to communicate with each other.  For example,

Good question. Unfortunately there's no general rule that you can apply 
and end up with an undisputably perfect solution.

Classes should communicate on a need-to-know basis. Take for example a 
RSS feed reader application. You may have a class representing a feed 
and a class representing a post. The feed will know what posts it 
contains, but the post probably won't know what feed it comes from. The 
interface would display a list of feeds (without knowing their 
contents), a list of posts within a feed (this needs to know both feed 
and feed contents) and the contents of a single post (knows only about 
an individual post).

 > I have created an Args class that sets a variety of internal
 > variables (__filename, __outputdir etc) by parsing the argv

Be careful with classes that simply act as a container for what are in 
fact global variables. A class should do one thing only (of course what 
you accept as 'one thing' is open for debate) and encapsulate all that's 
necessary for that particular thing. Make sure you're not 
overcomplicating your solution by making classes where they're not 
really necessary.

 > array from th command line.  What would be the preferred
 > mechanism for returning or passing along those variables

In some cases only some parts of the information contained in class A 
are relevant to class B - you should pass only that particular 
information, e.g. in the constructor or by setting a property of B. In 
your example, if you have a Reader class that is interested in the 
filename, you would not pass the whole Args object to it - only the 
filename, like this:

     myreader = Reader(Args.FileName)

 > to another class?  Maybe by a function method that returns
 > all of those variables?

Use properties if you need getter/setter methods or simple attributes 
otherwise. In your case, I would not make __filename etc. 'private' 
(that's what the double underscore suggests), then write a getter method 
for it - just call it FileName and be done with it. Python idiom here is 
more flexible than other languages.

-- 
Yours,

Andrei

=====
Mail address in header catches spam. Real contact info:
''.join([''.join(s) for s in zip(
"poet at aao.l pmfe!Pes ontuei ulcpss  edtels,s hr' one oC.",
"rjc5wndon.Sa-re laed o s npbi ot.Ira h it oteesn edt C")])