This page is to aid debugging unhelpful error messages.
<<TableOfContents>>
= AttributeError: 'NoneType' object has no attribute 'poll' =

{{{
Running canonical.testing.layers.TwistedAppServerLayer tests:
  Set up canonical.testing.layers.ZopelessLayer in 3.350 seconds.
  Set up canonical.testing.layers.DatabaseLayer in 0.457 seconds.
  Set up canonical.testing.layers.LibrarianLayer in 4.786 seconds.
  Set up canonical.testing.layers.LaunchpadLayer in 0.000 seconds.
  Set up canonical.testing.layers.LaunchpadScriptLayer in 0.001 seconds.
  Set up canonical.testing.layers.LaunchpadZopelessLayer in 0.000 seconds.
  Set up canonical.testing.layers.TwistedLaunchpadZopelessLayer in 0.000 seconds.
  Set up canonical.testing.layers.TwistedAppServerLayer in 31.229 seconds.
  Running:
    test_lock_with_magic_id (canonical.codehosting.puller.tests.test_scheduler.TestPullerMasterIntegration)Traceback (most recent call last):
  File "./test.py", line 190, in ?
    result = testrunner.run(defaults)
  File "/home/mwh/canonical/checkouts/trunk/lib/zope/testing/testrunner.py", line 238, in run
    failed = not run_with_options(options)
  File "/home/mwh/canonical/checkouts/trunk/lib/zope/testing/testrunner.py", line 403, in run_with_options
    setup_layers, failures, errors)
  File "/home/mwh/canonical/checkouts/trunk/lib/zope/testing/testrunner.py", line 585, in run_layer
    return run_tests(options, tests, layer_name, failures, errors)
  File "/home/mwh/canonical/checkouts/trunk/lib/zope/testing/testrunner.py", line 513, in run_tests
    test(result)
  File "/home/mwh/canonical/checkouts/trunk/lib/twisted/trial/unittest.py", line 632, in __call__
    return self.run(*args, **kwargs)
  File "/home/mwh/canonical/checkouts/init-stack-pull/lib/canonical/codehosting/puller/tests/test_scheduler.py", line 681, in run
    return TrialTestCase.run(self, result)
  File "/home/mwh/canonical/checkouts/trunk/lib/twisted/trial/unittest.py", line 961, in run
    result.stopTest(self)
  File "/home/mwh/canonical/checkouts/trunk/lib/twisted/trial/unittest.py", line 1158, in stopTest
    self.original.stopTest(method)
  File "/home/mwh/canonical/checkouts/trunk/lib/zope/testing/testrunner.py", line 871, in stopTest
    self.testTearDown()
  File "/home/mwh/canonical/checkouts/trunk/lib/zope/testing/testrunner.py", line 755, in testTearDown
    layer.testTearDown()
  File "/home/mwh/canonical/checkouts/init-stack-pull/lib/canonical/testing/profiled.py", line 28, in profiled_func
    return func(cls, *args, **kw)
  File "/home/mwh/canonical/checkouts/init-stack-pull/lib/canonical/testing/layers.py", line 1535, in testTearDown
    LayerProcessController.postTestInvariants()
  File "/home/mwh/canonical/checkouts/init-stack-pull/lib/canonical/testing/profiled.py", line 28, in profiled_func
    return func(cls, *args, **kw)
  File "/home/mwh/canonical/checkouts/init-stack-pull/lib/canonical/testing/layers.py", line 1389, in postTestInvariants
    if cls.appserver.poll() is not None:
AttributeError: 'NoneType' object has no attribute 'poll'
}}}

'''Solution:''' probably one of "make build", "make schema" or "killing left over librarians".

= AttributeError: 'thread._local' object has no attribute 'interaction' =

Example:
{{{
 Error in test lp.registry.tests.test_distroseries.TestDistroSeriesGetQueueItems.test_get_queue_items
 Traceback (most recent call last):
   File "/home/cjwatson/src/canonical/launchpad/lp-branches/queue-filter-source-bug-33700/lib/lp/testing/__init__.py", line 322, in run
     testMethod()
   File "/home/cjwatson/src/canonical/launchpad/lp-branches/queue-filter-source-bug-33700/lib/lp/registry/tests/test_distroseries.py", line 261, in test_get_queue_items
     pub_source = self.getPubSource(sourcename='alsa-utils')
   File "/home/cjwatson/src/canonical/launchpad/lp-branches/queue-filter-source-bug-33700/lib/lp/soyuz/tests/test_publishing.py", line 159, in getPubSource
     spn = getUtility(ISourcePackageNameSet).getOrCreateByName(sourcename)
 AttributeError: 'thread._local' object has no attribute 'interaction'
}}}

'''Solution:''' Call `login()`. This error often happens when trying to use core Launchpad objects when not logged in.

Most of the time `login(ANONYMOUS)` is good enough. `login` & `ANONYMOUS` should be imported from `lp.testing`. 

If you get this error when trying to use the `LaunchpadObjectFactory`, you should consider making your test a subclass of `TestCaseWithFactory`.