dynamic class instantiation

[Larry Bates]

Ognen Duzlevski wrote:
> Larry Bates <larry.bates at websafe.com> wrote:
>>> Now I want to use something like xml.dom.minidom to "parse" the 
>>> .xml file into a set of classes defined according to the "language 
>>> definition" file. The parse() method from the xml.dom.minidom 
>>> package will return a document instance and I can get the node 
>>> name from it. Say I got "page" as a string. How do I go about 
>>> instantiating a class from this piece of information? To make it 
>>> more obvious how do I create the page() class based on the "page" 
>>> string I have? I want to make this generic so for me the language 
>>> definition file will contain pages, functions, datasets etc. but 
>>> for someone else mileage will vary.
>>>
>>> Thanks,
>>> Ognen
> 
>> I think you should rethink this approach.  Why have 3 different
>> files instead of just defining everything as Python classes?
>> Get good baseclass definitions and use OOP inheritance to
>> create your specific class instances.  I think mixing XML,
>> language definition file (LDF), LDF to python parser is a LOT
>> harder (and slower) than just writing the classes outright.
> 
> Hi Larry,
> 
> here is the whole story. I have a database (postgres) xml "transformer"
> and substitution engine. It is written entirely in plpgsql and the whole 
> environment works something like this. Someone writes an xml file that
> defines a web page. This xml file is then fed into the database at the
> time of development. Now at runtime a mod_python apache webapp makes a 
> request for a certain page. Its xml form is retrieved from the database
> and a lot of data substitution goes on. Also parts of the page are "masked"
> out by the database engine based on the "level" of the person requesting
> the page. The "transformed and substituted" xml is then spit back to the 
> webapp which now applies an xml->html transformation and presents the
> final result to the user. So, I want people to be able to "define" their
> own "language" when it comes to page definitions. And instead of rewriting
> the xml file parser I want to be able to generate the parser.
> 
> Thanks,
> Ognen

Well you certainly can do that, but why invent a new language to define
the pages instead of just using Python directly?  I did an application
a few years ago that sounds a little like what you describe and now
wish I had not spent so much time doing Text File->Python transforma-
tions.  The logic got pretty ugly and extending things was rather
involved.  It wasn't until I started using ReportLab that I began to
"see the light" on just keeping everything in Python and inheriting
from base classes and extending.

If you want to define a Python class, the best way to do that is to
use Python from the outset.  I probably don't completely understand
what you want to do, but thought I'd at least put in my 2-cents worth.

Regards,
Larry