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 -- http://mail.python.org/mailman/listinfo/python-list