En Mon, 29 Dec 2008 19:47:51 -0200, Carl Banks <[email protected]>
escribió:
On Dec 29, 10:51 am, Kottiyath <[email protected]> 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)
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.
--
Gabriel Genellina
--
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