Fixtures is a tool that defines a Python contract for reusable state / support logic, primarily for unit testing. Helper and adaption logic is included to make it easy to write your own fixtures using the fixtures contract. Glue code is provided that makes using fixtures that meet the Fixtures contract in unittest compatible test cases easy and straight forward.
Why Fixtures
Standard Python unittest.py provides no obvious method for making and reusing state needed in a test case other than by adding a method on the test class. This scales poorly - complex helper functions propogating up a test class hierarchy is a regular pattern when this is done. Mocking while a great tool doesn't itself prevent this (and helpers to mock complex things can accumulate in the same way if placed on the test class).
By defining a uniform contract where helpers have no dependency on the test class we permit all the regular code hygiene activities to take place without the distorting influence of being in a class hierarchy that is modelling an entirely different thing - which is what helpers on a TestCase suffer from.
About Fixtures
A Fixture represents some state. Each fixture has attributes on it that are specific to the fixture. For instance, a fixture representing a directory that can be used for temporary files might have a attribute 'path'.
Creating Fixtures
Minimally, subclass Fixture, define setUp to initialize your state and schedule a cleanup for when cleanUp is called and you're done:
>>> import unittest
>>> import fixtures
>>> class NoddyFixture(fixtures.Fixture):
... def setUp(self):
... super(NoddyFixture, self).setUp()
... self.frobnozzle = 42
... self.addCleanup(delattr, self, 'frobnozzle')
This will initialize frobnozzle when setUp is called, and when cleanUp is called get rid of the frobnozzle attribute.
There is a helper for adapting a function or function pair into Fixtures. it puts the result of the function in fn_result:
>>> import os.path
>>> import shutil
>>> import tempfile
>>> def setup_function():
... return tempfile.mkdtemp()
>>> def teardown_function(fixture):
... shutil.rmtree(fixture)
>>> fixture = fixtures.FunctionFixture(setup_function, teardown_function)
>>> fixture.setUp()
>>> print os.path.isdir(fixture.fn_result)
True
>>> fixture.cleanUp()
The Fixture API
The example above introduces some of the Fixture API. In order to be able to clean up after a fixture has been used, all fixtures define a cleanUp method which should be called when a fixture is finished with.
Because its nice to be able to build a particular set of related fixtures in advance of using them, fixtures also have define a setUp method which should be called before trying to use them.
One common desire with fixtures that are expensive to create is to reuse them in many test cases; to support this the base Fixture also defines a reset which calls self.cleanUp(); self.setUp(). Fixtures that can more efficiently make themselves reusable should override this method. This can then be used with multiple test state via things like testresources, setUpClass, or setUpModule.
When using a fixture with a test you can manually call the setUp and cleanUp methods. More convenient though is to use the included glue from fixtures.TestWithFixtures which provides a mixin defining useFixture (camel case because unittest is camel case throughout) method. It will call setUp on the fixture, call self.addCleanup(fixture) to schedule a cleanup, and return the fixture. This lets one write:
>>> import testtools
>>> import unittest
Note that we use testtools TestCase here as we need to guarantee a TestCase.addCleanup method.
>>> class NoddyTest(testtools.TestCase, fixtures.TestWithFixtures):
... def test_example(self):
... fixture = self.useFixture(NoddyFixture())
... self.assertEqual(42, fixture.frobnozzle)
>>> result = unittest.TestResult()
>>> _ = NoddyTest('test_example').run(result)
>>> print result.wasSuccessful()
True
Fixtures implement the context protocol, so you can also use a fixture as a context manager:
>>> with fixtures.FunctionFixture(setup_function, teardown_function) as fixture:
... print os.path.isdir(fixture.fn_result)
True
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