decorator to fetch arguments from global objects

[andrea crotti]

2013/6/18 Wolfgang Maier <wolfgang.maier at biologie.uni-freiburg.de>

> andrea crotti <andrea.crotti.0 <at> gmail.com> writes:
>
> >
> >
> > 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)
> >         ...
> >
> > 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
> >
> > Now it kinda of works already with the decorator below.
> > The problem is that the argument is positional so I end up maybe passing
> it twice.
> > So I have to enforce that 'db' if there is passed as first argument..
> >
> > It would be a lot easier removing the db from the arguments but then it
> would look too magic and I didn't want to change the signature.. any other
> advice?
> >
> > def with_optional_db(func):
> >     """Decorator that sets the database to the global current one if
> >     not passed in or if passed in and None
> >     """
> >      <at> wraps(func)
> >     def _with_optional_db(*args, **kwargs):
> >         func_args = func.func_code.co_varnames
> >         db = None
> >         # if it's defined in the first elements it needs to be
> >         # assigned to *args, otherwise to kwargs
> >         if 'db' in func_args:
> >             assert 'db' == func_args[0], "Needs to be the first defined"
> >         else:
> >             db = kwargs.get('db', None)
> >
> >         if db is None:
> >             kwargs['db'] = get_current_db()
> >
> >         assert kwargs['db'] is not None, "Can't have a not defined
> database"
> >         ret = func(*args, **kwargs)
> >         return ret
> >
> >     return _with_optional_db
> >
>
> I'm not sure, whether your code would work. I get the logic for the db in
> kwargs case, but why are you checking whether db is in func_args? Isn't the
> real question whether it's in args ?? In general, I don't understand why
> you
> want to use .func_code.co_varnames here. You know how you defined your
> function (or rather method):
> class Entity:
>     def save_doc(db):
>         ...
> Maybe I misunderstood the problem?
> Wolfgang
>
>
>
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>


Well the point is that I could allow someone to not use "db" as argument of
the function if he only wants to use the global db object..

Or at least I want to check that it's the first argument and not in another
position, just as a sanity check.

I might drop some magic and make it a bit simpler though, even the default
argument DEFAULT_DB could be actually good, and I would not even need the
decorator at that point..
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