subclassing versus object redefinition

[wheres pythonmonks]

Roald:

First, I must admit, I didn't know I could create an ABC in python.
Now I see (http://docs.python.org/library/abc.html). Thank you.

I think that the crux of the matter is in points #3,  #4, and #5 that
you raised:

3) adding stuff to instances is less reusable that adding stuff to (sub)classes
4) if I'm reading your code and want to know what an object is like, I look
 at the class, not through your whole program to collect all bits and
pieces of information spread out over it

On #3:  Not clear that all possible specializations warrant
factorization into a class.  Indeed, this may result in "premature
abstraction" -- and make the code less clear.  Also, it will freeze in
the base classes, making future refactoring a headache.

On #4:  Unless I misunderstood something, there is nothing in python
that ensures that a class definition is localized.  So, putting
definitions in classes, does not guarantee that the definition is at a
single location in the code.

5) why would you want a thinner class hierarchy?

The yo-yo anti-patten:
http://en.wikipedia.org/wiki/Yo-yo_problem

I have a pretty strong preference for using a small number of useful
objects, instead of having code littered with objects strewn across
the namespace.

Maybe there is a Python ABC tutorial out there that can enlighten me?

W

On Tue, Aug 3, 2010 at 10:06 AM, Roald de Vries <downaold at gmail.com> wrote:
> On Aug 3, 2010, at 2:46 PM, wheres pythonmonks wrote:
>>
>> Hi!
>>
>> I have a class (supposed to be an abstract base class):
>> In python (as opposed to static languages like C++) I don't seed to
>> subclass the base class, but instead I can simply override the
>> behavior of stub methods and values.
>> Is there a preference between between subclassing (C++ approach) and
>> overriding methods/data in an instance?  From a design perspective?
>> I think I prefer the override/redefine approach because it result in a
>> thinner class hierarchy.
>>
>> It seems like inheriting an ABC is needed only when I must share
>> instances (between multiple parts of the code, or if the subclass is
>> instantiated in different places...)
>>
>> Thoughts?
>
> 1) some things are just not possible in instances, like overriding operators
> 2) abstract base classes are not supposed to be instantiable, so if you are
> able to do it anyway, that is a hack
> 3) adding stuff to instances is less reusable that adding stuff to
> (sub)classes
> 4) if I'm reading your code and want to know what an object is like, I look
> at the class, not through your whole program to collect all bits and pieces
> of information spread out over it
> 5) why would you want a thinner class hierarchy?
>
> So I would go for inheritance.
>
> Cheers, Roald
>
>