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Within You Without You

·3 mins

Testing is hard, writing testing frameworks is easy. In an effort to make testing easier, big projects like Twisted, Bazaar and Zope write their own testing frameworks. That way they control both the test runner and the tests that are run. It’s actually quite convenient.

However, it’s led to a significant problem:

There are many similar implementations of xUnit in Python, each with subtle incompatibilities.

Running Twisted tests in the Zope test runner? Watch out for the threads that the reactor maintains between tests. Running Bazaar tests with Trial? On my machine, I get told that elementtree doesn’t have an ‘ElementTree’ attribute. Hmm.

When talking about this problem, I often refer loosely to “PyUnit compatibilty”. The idea is that:

  1. Every Python test runner should support running vanilla Python standard library unittest.TestCase tests.
  2. Every Python unit test should be able to be run using the mechanisms in in the Python standard library.

In other words, this code should Just Work:

import unittest
from yourframework import testing

class PythonTestCase(unittest.TestCase):
    def test_something(self):

class FrameworkTestCase(testing.TestCase):
    def test_something(self):

if __name__ == '__main__':
    python_test_result = unittest.TestResult()
    framework_test_result = testing.TestResult()
    # At this point, python_test_result and framework_test_result
    # should hold equivalent data.

If your framework is PyUnit compatible then the above fragment should give the same results if run directly or if run in your runner. Things get a little bit hazier when it comes to test discovery.

So, if your unit test requires that it be run inside a special suite (e.g. TrialSuite) in order to work correctly, it is not PyUnit compatible. If your test runner does some critical set up that enables features that your tests need, then it is not PyUnit compatible.

This leads to a kind of thinking where certain features belong on the base test case and others belong in the test runner. Putting features in the wrong place might not lead to a strict incompatibility, but it can lead to significant inconvenience. (And what are automated tests if not a convenience?).

Two examples from Twisted:

Temporary Working Directory

Trial creates a _trial_temp working directory and changes into that directory to run tests. In Trial, this feature is provided by the test runner. It should be provided by the base TestCase class.

  • It’s not clear that every test needs this feature.
  • Twisted tests now assume that they can create files with impunity. When Twisted tests are run in a different test runner, they leave garbage files everywhere.


By default, any Trial test that runs for more than two minutes will fail with a timeout error. The timeout period can be configured on a per-test, per-test-class, per-module or per-package basis. Trial implements this feature on TestCase, it should be implemented on the runner.

  • Even in the Twisted test runner, this makes debugging more painful. You must do all of your debugging in under two minutes.
  • Intuitively, you might think that the runner should control how tests are run.
  • Tests that don’t descend from Trial’s TestCase can still hang.
  • Two minutes might be good enough for me on a Monday, but I might be busier on Friday. I should be able to change the timeout without changing code.