Trial is Twisted's unit testing system, an extension of Python's `unittest` module.

Trial is two things. The first is a command-line test runner, which can be run on plain Python unit tests. It's very simple to use. For example, the fastest way to get going with Trial is to write a plain Python unit test like this:

import unittest
class DemoTest(unittest.TestCase):
    def test_passes(self):
    def test_fails(self):"I failed.")

and then run it, like this:

$ trial 
    test_fails ...                                                       [FAIL]
    test_passes ...                                                        [OK]

Traceback (most recent call last):
  File "unittest/", line 318, in run
  File "", line 9, in test_fails"I failed.")
  File "unittest/", line 393, in fail
    raise self.failureException(msg)
exceptions.AssertionError: I failed.

Ran 2 tests in 0.316s

FAILED (failures=1, successes=1)

Trial's test runner can do automated unit-test discovery across files, modules, or even arbitrarily nested packages. For example, you can run Twisted's own tests with it by just typing trial twisted in a shell.

Trial's test runner will also automatically catch errors that are logged but not explicitly handled. For example, this test will fail:

from twisted.python import log
import unittest
class DemoTest(unittest.TestCase):
    def test_failWithLog(self):
            1 / 0

Trial will also catch certain types of global state manipulations, like leaving stale sockets around in the global Twisted reactor, and fail tests as a result.

For a more complete list of the test runner's features, trial --help will provide options. A selection of features which can be enabled with these options include:

  • Colored output: red for failure, green for success (even on Windows!)
  • support for the subunit test-result protocol
  • command-line control over the garbage collector (optionally collect between each test, or disable it)
  • test-order randomization, based on a random seed so the same order can be repeated
  • run your tests in a loop until they fail
  • produce coverage reports
  • real-time error reporting and traceback logging - useful for when your tests hang!
  • run your tests under PDB and automatically break into the debugger on failure

Trial also provides a plug-in interface for customizing and recording the output of test runs. You can type trial --help-reporters for more information about the plugins that are available on your system.

In addition to the test runner, Trial also includes a test library, derived from Python's unittest.TestCase. You can use this library by subclassing twisted.trial.unittest.TestCase rather than Python's built-in unittest.TestCase.

The main unique feature of this testing class is the ability to return a Deferred from a test-case method. A test method which returns a Deferred will not complete until that Deferred has fired; the reactor will be automatically set up and run for you, along with a timeout to make sure that tests don't run forever. If the Deferred fires successfully, the test passes. If it fires with an error, the test fails. This makes it possible to easily unit-test asynchronous event-driven code, or to use Twisted APIs that return Deferreds in order to write automated functional tests that communicate with a live running service.

Trial's TestCase also includes several convenience features which we find useful for testing Twisted itself; if you're writing Twisted code, it's likely you'll find them useful too.


Trial is a core component of Twisted. As such, there is no need to download it separately. If you have downloaded the Twisted tarball, you'll find the Trial source code in the twisted/trial directory.


You can find a thorough tutorial on how to use Trial to do test-driven development with Twisted here.

You can find Trial API documentation here. If you run trial --help from the command line, you'll see a list of possible invocation arguments (the trial command can be found in bin in the tarball). Beyond, seeing as much of Twisted itself is of course tested with Trial, you can examine the test directories of the various Twisted projects to see examples of Trial use. And as ever, having a look through the source code is also instructive.


See TrialDevNotes for thoughts on future developments.

Open Issues


The maintainer of Trial is JonathanLange. The best way to get your bug fixed is to file it in the tracker (see the View Tickets and New Ticket links above).

Last modified 10 years ago Last modified on 12/16/11 00:55:35