An attempt to use a python-based mini declarative language for form definition

[Thomas Heller]

Carlos Ribeiro <carribeiro at gmail.com> writes:

> I'm doing some experiments with mini declarative languages (as
> explained by David Mertz in
> http://www-106.ibm.com/developerworks/library/l-cpdec.html) in Python,
> with the intention to use it as the mechanism to define data entry
> forms. My final goal is to have a simple approach to automatic
> generation of visual interfaces. The complete framework is rather big,
> so let's us focus at this specific problem.
>
> -- I would like to describe my data entry forms with plain Python
> code. I don't want to use XML, dicts or other data-driven solution;
> not because I don't like it, not because I don't know about it, only
> because I want to try a different approach.
>
> -- This is a simple code snippet of the intended form declaration:
>
> class UserForm(Form):
>     nickname = TextBox(length=15, default="")
>     password = TextBox(length=10, default="", password=True)
>     name     = TextBox(length=40, default="")
>
> It's actually based to some extent on Ian Bicking's sqlobject library,
> that uses a similar approach to build entity definitions. But there's
> a catch: the class constructor receives a dict, and has no way to tell
> the original ordering of the attributes in the original class. The
> field names are passed in an arbitrary ordering, due to the use of the
> dict mapping.
>
> -- I've tried using metaclasses or other similar magic; I've read the
> tutorials, tried some code, and read sqlobject own implementation.
> This is not a problem for sqlobject, because the order of the columns
> in the database is totally isolated from the object representation.
> But in my case, it is a problem, because I need the fields to be in
> the correct order in the display.
>
> My question is, there is any way to retrieve the class attributes in
> the order they were declared? I could not find any; __setattr__ won't
> work because the dict is constructed using the native dict type before
> __new__ has a chance at it. Is there anything that I'm overlooking?

No, there is no way.  But there is a trick you can use (I've played with
stuff like this, in a different context, in the past): You can use an
instance variable or global in the TextBox callable, that is incremented
on each call.  This counter is somehow attached to the object that
TextBox returns, and lets you order these objects afterwards.  Makes
sense?

Thomas