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From: Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de>
Subject: Re: decorator to fetch arguments from global objects
Date: Wed, 19 Jun 2013 07:39:39 +0000 (UTC)
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Xref: csiph.com comp.lang.python:48692

andrea crotti <andrea.crotti.0 <at> gmail.com> writes:
 
> 
> 2013/6/18 Terry Reedy <tjreedy <at> udel.edu>
> 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 missed a self it's a method actually..
>          ...
> 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')
> 
> 
> Yes that's exactly why I want a decorator, to avoid all this boilerplate
for every function method that uses a db object.. 
> 

The standard way of avoiding boilerplate like this is not to use a
decorator, but a regular function/method that you call from within your
code. Just encapsulate Terry's code into a module-level function (or a
static class method):

def guarantee_dbobject(db):
    if db is None:
        db = fetch_from_global()
    elif not isinstance(db, dbclass):
        raise TypeError('need dbobject')
    return db

Then use it as a simple one-liner like this:
def save_doc(self, db=None):
    db = guarantee_dbobject(db)
    ...

It's as much effort to use this as decorating the method, but I think it's
clearer and it avoids the potential problems that a decorator causes with
introspection (which might not be a problem now, but could turn into one at
some point).
Wolfgang