[TIP] setUp and tearDown behavior

Alfredo Deza alfredodeza at gmail.com
Tue Jan 19 11:23:31 PST 2010

On Tue, Jan 19, 2010 at 9:55 AM, holger krekel <holger at merlinux.eu> wrote:

> Hi Alfredo,
> On Mon, Jan 18, 2010 at 21:37 -0500, Alfredo Deza wrote:
> > I am building some tests for a Python installer that copies some files
> and
> > changes some permissions.
> >
> > Nothing weird there.
> >
> > However, in the test class I am starting with a setUp that basically
> calls
> > the installer (so the tests can see if everything is where it should be),
> > this is followed by a few assertions and finally a tearDown is called
> where
> > everything gets uninstalled.
> >
> > When running the tests with nosetests, I see that setUp and tearDown are
> > called for *every* method in my test class (e.g. installs => runs test
> > method => uninstalls ....)
> >
> > Isn't setUp supposed to be run once at the beginning of the class? or is
> > this expected?
> >
> > In case this is expected, is there a way to do it *just once *for all the
> > methods?
> The others gave various answers related to unittest and its extensions -
> i'd like to add another way using py.test "funcargs".  The main advantage
> is that you can completely decouple test fixture and actual test code,
> allowing
> for more flexibility and ease of doing functional/system testing.
> Would that be specific to py.test ? I am trying to build agnostic tests
that can be run with anything
and understood in the original way it was written.

> Example: you write one or multiple test functions that look like this
> (also can be put on a class of course):
>    def test_installer(setupdir):
>        # do your tests, setupdir is your fixture state
> This is a test function that runs and works with a "setupdir" function
> argument.
> You define a function argument factory which is invoked to provide
> an instance of it:
>    def pytest_funcarg__setupdir(request):
>        return request.cached_setup(scope="session",
>            setup=lambda: MySetupDir(),
>            teardown=lambda mysetupdir: mysetupdir.finish()
>        )
> The specified 'setup' and 'teardown' will be called exactly once per
> session.  If you don't run a test that needs the funcarg the factory
> function will not be called at all.  In any case, your test functions
> can stay completely ignorant from how fixtures are managed.
> You can change scoping by modifying the factory function - e.g.
> add a command line option that forces "function" scoping which usually
> avoids "cascading failures" and provides better test isolation at the
> expensve
> of longer test run durations.  Useful for a nightly Continous Integration
> configuration.
> Lastly, the 'request' object is a handle to the particular test
> function - it provides access to the test function object, its module,
> class object etc. and you can thus easily mark a test to receive a
> specially parametrized "setupdir" ...  see http://tinyurl.com/yfw82l5
> for details.
> cheers,
> holger

Alfredo Deza
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