Timing-Out a Deferred

Twisted’s defer.addTimeout allows a time limit to be set for the resolution of a Deferred. If the Derferred does not resolve in the specified time, it will be Errback-ed with a CancelledError.

When it is useful

Deferred objects are not guaranteed to resolve in any concrete amount of time, but often after a reasonable time limit is reached it is reasonable to assume that something has gone wrong and that the Deferred should be cancelled with an error.

As an example, consider an attempt to connect to a remote server. If the connection is taking too long to become established, it may be better to simply cancel the attempt and try again with a different server.

How to use it


To add a Timeout to a Deferred, simply call addTimeout() on the Deferred object.

Below, inside of our unit test, we create a Deferred d and call its addTimeout function, which automatically cancels it in 5 seconds and uses the reactor to track the passage of time before cancellation.

Deferreds cancelled by addTimeout raise a TimeoutError AND, as they call cancel, a CancelledError as well. We have added assertions to our unit test to confirm this behaviour:

from twisted.internet import reactor
from twisted.internet.defer import TimeoutError, CancelledError, Deferred
from twisted.trial import unittest

class DeferredExampleTest(unittest.TestCase):

    def testTimeOutIn5(self):
        def timeOutIn5() -> Deferred:
            d = Deferred()
            d.addTimeout(5, reactor)
            return d

        # A timed-out Deferred should always raise a TimeoutError
        self.assertRaises(TimeoutError, self.addCleanup(timeOutIn5))
        self.assertRaises(CancelledError, self.addCleanup(timeOutIn5))

Sample Output

Ran 1 test in 10.013s