[Python-ideas] Test Class setup and teardown in unittest

[Jonathan Lange]

Mark Roddy <markroddy at ...> writes:
> 
> Earlier this week on the Testing In Python list, there was a
> discussion on how to execute a setup and/or teardown for a single test
> class instead of for each test fixture on the class (see the 'setUp
> and tearDown behavior' thread).  I have had to deal with situation
> myself before, and I am obviously not the only one (since I did not
> initiate the thread).  As such I'd like to propose adding a class
> level setup and tear down method the unittest TestCase class.
> 
> Rationale:
> Test cases can at times require the setup of expensive resources.
> This is often the case when implementing integration testing.  Having
> to perform this setup for each fixture can prohibited for large number
> of fixtures and/or for resources that are expensive to setup.  For
> example, I have several hundred integration tests that hit a live
> database.  If I create the connection object for each fixture, most of
> the tests fail due to the maximum connection limit being reached.  As
> a work around, I create a connection object once for each test case.
> Without such functionality built in, the common idiom runs along these
> lines:
> 
> class MyTest(TestCase):
> 
>     def setUp(self):
>         if not self.ClassIsSetup:
>             self.setupClass()
>             self.ClassIsSetup=True
> 
> While this achieves the desired functionality, it is unclear due to
> conditional setup code and is also error prone as the same code
> segment would need to appear in every TestCase requiring the
> functionality. 

I agree that this is a common problem, and that the Python community would 
benefit from a well-known, well-understood and widely applicable solution.

> Having a class wide setup and teardown function that
> implementers of test cases can override would make the code/intent
> more clear and alleviate the need to implement test case functionality
> when the user should be focusing on writing tests.
>

I'd take issue with the argument that this makes the intent clearer. In your 
motivating example, what you mean is "the test needs a connection" not "I want 
to do a set up that spans the scope of this class". A better approach would be 
to _declare_ the dependency in the test and let something else figure out how to 
best provide the dependency.

Also, setUpClass / tearDownClass is probably a bad idea. We implemented such 
behaviour in Twisted's testing framework some time ago, and have only recently 
managed to undo the damage.[1]

If you do this in such a way that setUpClass methods are encouraged to set 
attributes on 'self', then you are compelled to share the TestCase instance 
between tests. This runs contrary to the design of unittest, and breaks code 
that relies on a test object represent a single test.

It turns out that if you are building extensions to testing frameworks (like any 
big app does), it helps a lot to have an object per runnable test. In 
particular, it makes distributing tests across multiple processors & multiple 
computers much, much easier.

It also poses a difficult challenge for test runners that provide features such 
as running the tests in a random order. It's very hard to know if the class is 
actually "done" and ready to be torn down.

Finally, we found that it's use often lead to hard-to-debug failures due to test 
isolation issues.

There are already alternatives for this in the Python unit testing world. 
zope.testing provides a facility called 'layers' which solves this problem. I 
don't like it[2], but if we are talking about changing the standard library then 
we should consult existing practice.

Another solution is testresources[3]. It takes a declarative approach and works 
with all the code that's already in the Python standard library.

I'm not deeply familiar with xUnit implementations in other languages, but the 
problem isn't unique to Python. I imagine it would be worth doing some research 
on what Nunit, JUnit etc do.
 
> I emailed Michael Foord about some of his comments in the TIP thread
> and to ask if he would be interested in a patch adding this
> functionality, and I have included his response below.  I would like
> to hear people's comments/suggestions/ideas before I start working on
> said patch.
> 

Replies below.

...
> Michael Foord's Email:
> =======================================
> I would certainly be interested in adding this to unittest.
> 
> It needs a discussion of the API and the semantics:
> 
> * What should the methods be called? setup_class and teardown_class or
> setupClass and teardownClass? For consistency with existing methods
> the camelCase should probably be used.

In Twisted, we called them setUpClass and tearDownClass.

> * If the setupClass fails how should the error be reported? The
> *easiest* way is for the failure to be reported as part of the first
> test

Of course, it actually represents a failure in all of the tests. Another way of 
doing it is to construct a test-like object representing the entire class and 
record it as a failure in that.

> * Ditto for teardownClass - again the easiest way is for it to be
> reported as a failure in the last test

Ditto.

> * If setupClass fails then should all the tests in that class be
> skipped? I think yes.
> 

They should all be failed.

> Also details like ensuring that even if just a single test method from
> a class is run the setupClass and teardownClass are still run. It
> probably needs to go to python-dev or python-ideas for discussion.
> 

That's really really hard, and not a detail at all. See above.

I know this is a mostly negative response, but I really do hope it helps.

jml

[1] http://twistedmatrix.com/trac/ticket/4175
[2] http://code.mumak.net/2009/09/layers-are-terrible.html
[3] http://pypi.python.org/pypi/testresources/