Python module import loop issue

[Carl Banks]

Gabriel Genellina wrote:
> En Mon, 29 Dec 2008 19:47:51 -0200, Carl Banks <pavlovevidence at gmail.com>
> escribi�:
> > On Dec 29, 10:51�am, Kottiyath <n.kottiy... at gmail.com> wrote:
>
> >> Module Factory:
> >> A1Factory: {'B1Tag':1.1.B1, 'C1Tag':1.2.C1, 'D1Tag':1.3.D1'}
> >> A2Factory: {'B2Tag':2.1.B2, 'C2Tag':2.2.C2, 'D2Tag':2.3.D2'}
> >>
> >> But, since Module requires objects of B1, C1 etc, it has to import
> >> Factory.
> >> Now, there is a import loop. How can we avoid this loop?
> >
> > I'm going to suggest three ways: a straightforward, good-enough way; a
> > powerful, intelligent, badass way; and a sneaky way.
>
> In Python 2.6 (and 3.0) there is a fourth way: class decorators.
>
> > 1. The straightforward, good-enough way
> >
> > Define functions in Factory.py called register_A1_subclass and
> > register_A2_subclass, then call them whenever you create a new
> > subclass.
>
> Class decorators are a clean variant of this approach (in my opinion).
>
> > package1/module1.py:
> > -----------------------------
> > import Factory
> >
> > class B1(A1):
> >     # define class B1 here
> >
> > Factory.register_A1_subclass("B1Tag",B1)
> > -----------------------------
>
> That would become:
>
> @Factory.register_A1_subclass("B1Tag")
> class B1(A1):
>    ...
>
> (for an adequate variant of register_A1_subclass). The advantage is that
> the "register" stuff appears prominently near the name of the class, and
> there is no need to repeat the name.
> Also, "B1Tag" can be left out, if it is stored as a class attribute of B1
> (in some cases using __name__ is enough)

Thanks for the additional suggestion.


> > 2. The powerful, intelligent, badass way
> >
> > Metaclasses.  I would guess you do not want to do this, and I wouldn't
> > recommend it if you haven't studied up on how metaclasses work, but
> > it's a textbook example of their usefulness.  If you expect to use
> > factory functions like this a lot, it might be worth your while to
> > learn them.
>
> A problem with metaclasses is when you have intermediate subclasses that
> are not meant to be registered, but the metaclass applies equally to all
> of them.

Not the way I wrote it.  If you'll note, the metaclass only added the
class to the factory map if a tag attribute was defined in the class
dict.


Carl Banks