decorator to fetch arguments from global objects

[Terry Reedy]

On 6/18/2013 5:47 AM, andrea crotti wrote:
> Using a CouchDB server we have a different database object potentially
> for every request.
>
> We already set that db in the request object to make it easy to pass it
> around form our django app, however it would be nice if I could set it
> once in the API and automatically fetch it from there.
>
> Basically I have something like
>
> class Entity:
>       def save_doc(db)

If save_doc does not use an instance of Entity (self) or Entity itself 
(cls), it need not be put in the class.

>          ...
>
> I would like basically to decorate this function in such a way that:
> - if I pass a db object use it
> - if I don't pass it in try to fetch it from a global object
> - if both don't exist raise an exception

Decorators are only worthwhile if used repeatedly. What you specified 
can easily be written, for instance, as

def save_doc(db=None):
   if db is None:
     db = fetch_from_global()
   if isinstance(db, dbclass):
     save_it()
   else:
     raise ValueError('need dbobject')


-- 
Terry Jan Reedy