[pytest-dev] pytest-xdist-1.11: restarting crash nodes, fixture cache fix

[Andreas Pelme]

Hi Holger,

On 19 sep 2014, at 09:52, holger krekel <holger at merlinux.eu> wrote:
> I presume that with module and class scope people don't presume that
> a fixture survives until the end of a process so eager teardown is less
> of a problem, there, right?


I have not fully grasped how fixture teardown currently happens in pytest. To explore it, I came up with this example:

import pytest


@pytest.yield_fixture(scope='class')
def fix():
    print 'fixture setup'
    yield
    print 'fixture teardown'



class TestFoo():
    def test_a(self, fix):
        print 'test_a'

    def test_b(self):
        print 'test_b'

    def test_c(self, fix):
        print 'test_c'


It gives me the output (with the latest pytest from PyPI):

fixture setup
test_a
test_b
test_c
fixture teardown

I.e. even though test_b does not actively request the fixture, it is active during the test.

Is this even considered to be a bug or a feature? :-) This behavior may be considered a bug since it makes test suites brittle - if the fixture does not contain a value itself, it can probably be neglected to actually properly request the fixture, but test_b will still accidentally have “fix” available. Bugs like this will only show itself when running a subset of the test suite (and with xdist!).

I would prefer (I think) if all fixtures where torn down when they are not requested (in my example, before test_b is run, to ensure test_b is only run with its own fixtures).

However, if all teardowns worked like this, the efficiency will be very bad since there will be a lot of teardowns. I think these fixtures that should be allowed to stay alive, should be explicitly declared like that, like the option to pytest.fixture() suggested earlier.

I think this makes sense regardless of whether the fixture is class, module or session scoped. Having different semantics depending on the scope would be confusing. Fixture authors must be aware and decide whether or not a fixture may accidentally be available to wrong tests. I have a hard time to see how we can solve this properly without a flag or some kind of distinction for this.

I am probably missing a lot of details here, what are your thoughts on this?

Cheers,
Andreas