[Tutor] object design question

Alan Gauld alan.gauld at btinternet.com
Tue May 15 18:25:20 CEST 2007

"Kent Johnson" <kent37 at tds.net> wrote
>> Howdy,
>> I would be interested in some discussion of
>> which of the following approaches is preferred and why.
>> class RstManager:
>>     def __init__(self, text):
>>         self.parsed = parseRst(text)
>>     def parseRst(self, text):
>>         parsed = <do stuff to text>
>>         return parsed
> If you have no further need of text, I prefer the second. If you 
> need to
> keep text around then use the first one or possibly

I agree with Kent J. It depends a lot on what else your class is 
If it needs text or if it will parse reguilarly.

However I'm also always suspicious of a class called xxxManager.

Classes are by definition there to manage some data by
providing some behaviour. Usually the Manager bit can
be left off the name. Then the class is just Rst.

I don't know what an Rst is, but my guess is that you will
want one of them as an object rather than some kind of
manager object. To use a more mundane example:

class FileManager:
    def open(self, name): ....
    def close(self):....

fm = FileManager('foo.txt')

Do I really want a file manager object or do I want a file?
Which is the object? Often when we thinkmof objects as
managers we are really thinking in terms of some data
that we want to apply functions to. Really we should be
thinking of the class as representing the objects themselves.

If you do have a real manager object it is usual for it to contain
one or more of the things it manages, thius you'd expect
the definition to look something like:

class FooManager:
   def __init__(self, aFoo):

   def sort(self):....
   def find(self,aFooRef):....

ie The init takes a Foo as an argument and manages a
collection of Foos.

But I don't know your domain and you may well really mean
an RstManager. But its worth considering, it may well make
your code more intuitive to read.

If you want to read much more on this theme try and find
a copy of the book OOP by Coad and Nicola. They discuss
the problems that can arise in an OOP program with Manager


Alan Gauld
Author of the Learn to Program web site

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