Pre-Called Deferreds

Twisted allows users to create instances of ‘pre-called’ Deferreds by calling defer.succeed(someValue) or, this is a convenience feature that is the same as calling:

d = defer.Deferred()

for defer.succeed() or:

d = defer.Deferred()



Below we have created a pre-called Deferred, and a pre-failed Deferred:

from twisted.internet.defer import CancelledError, fail, succeed
from twisted.trial import unittest

class ExampleTests(unittest.TestCase):

    def testPreCalledSuccess(self):

        # Create a deferred that has already resolved
        deferredPreCalledSuccess = succeed("OK")

        # Create a Deferred and Cancel it
        deferredPreCalledFail = fail(1)

        # Returning a pre-cancelled Deferred fails with a CancelledError
        self.assertEqual("OK", deferredPreCalledSuccess.result)
        self.assertFailure(deferredPreCalledFail, int)

Running the code should result in the test passing similarly to below:

Ran 1 test in 0.151s


Our self.assertFailure function passing on our preCalledFail, would normally indicate that at some point during normal operation it had raised an exception and become a Failure instance.

Our preCalledSuccess having a result would normally indicate that it had resolved with a result of “OK”. In both cases we were able to skip past these steps and simply return a pre-fired Deferred.