"Nathan" == Nathan nathan.stocks@gmail.com writes:
Nathan> On Mon, Jun 30, 2008 at 4:20 PM, Terry Jones terry@jon.es wrote:
You're not using deferreds properly. In the simple/typical case, when you call a function that returns a deferred, you will want to add (at least) a
Nathan> I don't think that's true. Jean-Paul was the one who told me to do Nathan> it that way in the first place:
Nathan> http://twistedmatrix.com/pipermail/twisted-python/2008-April/017304.html
Sorry, but that doesn't seem related - you may have misinterpreted what Jean-Paul was saying.
Nathan> If your theory was true, I would be getting garbage everywhere I Nathan> return (and use) complicated objects, and I'd be getting True Nathan> everywhere that I return booleans.
I don't fully understand this last sentence. Try this:
from twisted.internet import defer d = defer.Deferred()
if d: print 'hey hey hey'
Python treats the deferred as true here. Exactly the same goes for your original code:
def check_db(): d = {some db query that returns a deferred} d.addCallback(handle_db_result) return d
def main_func(): if check_db(): # This eventually returns True or False # Do some stuff else: # Do some different stuff
The return result of check_db is a deferred. If you test it with an 'if' it's always going to be true. If you do this
def main_func(): d = check_db() print d if d: # Do some stuff else: # Do some different stuff
You'll see that d is a deferred.
In both code examples you sent (the pseudo and the working) you are not treating the return value of a function that returns a deferred as a deferred. In one example you're testing it in an 'if' and in the other you're printing it. You need to add a callback in both cases.
Terry
-
Terry Jones