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"Nathan" == Nathan <nathan.stocks@gmail.com> writes:

Nathan> On Mon, Jun 30, 2008 at 4:20 PM, Terry Jones <terry@jon.es> wrote:

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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