txdpce provides a Twisted Python class, DeferredParallelCommandExecutor.
It allows you to register competing functions that compute a result. The
execute method of the class calls them all one after another and returns a
Deferred that will fire with the first success. The deferreds corresponding
to the incomplete function calls are cancelled.  If all functions fail, the
deferred errs with a list of the failures.

The point is that you might have a couple of ways of getting a result (out
of a cache, off the network) and you want to try them all, taking the
answer from the first one that figures it out.

See http://blogs.fluidinfo.com/terry/2011/07/12/txdpce-a-twisted-class-for-deferred-parallel-command-execution/ for more of an explanation.