Module Structure/Import Design Problem

[Rafe]

On Nov 21, 1:39 am, Steve Holden <st... at holdenweb.com> wrote:
> Rafe wrote:
> > Hi,
>
> > I am in a situation where I feel I am being forced to abandon a clean
> > module structure in favor of a large single module. If anyone can save
> > my sanity here I would be forever grateful.
>
> > My problem is that classes in several modules share a common base
> > class which needs to implement a factory method to return instances of
> > these same classes.
>
> > An example to help illustrate what I mean:
> > Lets say I have the following modules with the listed classes:
> >  - baselib.py   with  BaseClass
> >  - types.py   with  TypeA, ...
> >  - special.py   with  SpecialTypeA, ...
>
> > Which would be used a bit like this:
> >>>> type_a = any_type_instance.get_type("TypeA")
> >>>> special_type = type_a.get_type("SpecialTypeA")
>
> > Again, I can get around this by dumping everything in to one module,
> > but it muddies the organization of the package a bit. This seems like
> > a problem that would come up a lot. Are there any design paradigms I
> > can apply here?
>
> Well a simple way to do this is to observe that even when a base class's
> method is inherited by an instance of a subclass, when the method is
> called the type of "self" is the subclass. And you can call the
> subclass's type to create an instance. Perhaps the following  code would
> make it more obvious:
>
> $ cat baseclass.py
> class Base(object):
>     def factory(self, arg):
>         return type(self)(arg)
>
> sholden at lifeboy /c/Users/sholden/Projects/Python
> $ cat subclass.py
> from baseclass import Base
>
> class sub(Base):
>   def __init__(self, arg):
>     print "Creating a sub with arg", arg
>
> s = sub("Manual")
>
> thing = s.factory("Auto")
> print type(thing)
>
> sholden at lifeboy /c/Users/sholden/Projects/Python
> $ python subclass.py
> Creating a sub with arg Manual
> Creating a sub with arg Auto
> <class '__main__.sub'>
>
> Hope this helps.
>
> regards
>  Steve
> --
> Steve Holden        +1 571 484 6266   +1 800 494 3119
> Holden Web LLC              http://www.holdenweb.com/

Hi Steve,

Correct me if I have this wrong, but the problem with your solution is
that it only creates a new instance of the same class, type(self),
while I need to return any number of different possibilities.

I thought about getting the module from self...
>>> class base(object):
>>>     def factory(self, type):
>>>         module = sys.modules[self.__class__.__module__]
>>>         return getattr(module, type)

...but my baseclass is used from several modules so this would be
inaccurate for me (the factory method only uses my 'types' module, so
a hard import works)

I'm still wondering what Arnaud meant by "make types register
themselves with the factory function"

- Rafe