[pytest-dev] Fixture ordering and scopes

[Brian Okken]

Bruno,

Please, merge that PR!
caveat: I have not reviewed the code.

However, ...
I get numerous questions about it, and I always tell people to create
artificial dependencies between fixtures that need to run in a certain
order.
The general mental model that people have for fixtures is that they are run
in scope order.
I think the current behavior of file order overriding scope order is insane
and a bug.

- Brian

On Thu, Mar 15, 2018 at 2:42 PM, Bruno Oliveira <nicoddemus at gmail.com>
wrote:

> Hi everyone and Holger,
>
> Looking at the code below:
>
>
> data = {}
> @pytest.fixture(scope='session')def clean_data():
>     data.clear()
> @pytest.fixture(autouse=True)def add_data():
>     data['value'] = 1
> @pytest.mark.usefixtures('clean_data')def test_foo():
>     assert data.get('value')
>
> Should test_foo fail or pass? Keep your answer in mind before proceeding.
> :)
> ------------------------------
>
> I ask this because *unrelated* fixtures, i.e. which don’t depend on each
> other, are executed in the order they are declared in the test function
> signature, regardless of scope.
>
> The example above *fails*, because the fixtures are executed in (add_data,
> clean_data) order: add_data, being *autouse*, is added to the beginning
> of the argument list, and afterwards clean_data is inserted because of
> the usefixtures mark.
>
> This came up in #2405 <https://github.com/pytest-dev/pytest/issues/2405>,
> where Jason Coombs assumed that clean_data, being session-scoped, would
> always be executed first.
>
> I wonder if the current state of things is by design, or just an accident
> of how things work?
>
> I opened up a PR <https://github.com/pytest-dev/pytest/pull/3306> which
> does sort parameters by scope while keeping the relative order of fixtures
> of same scope intact, and the test suite passes without failures so if the
> current behavior is by design there are not tests enforcing it. Using code
> in the PR, then things might also be surprising:
>
> @pytest.fixture(scope='session')def s(): pass
> @pytest.fixture(scope='function')def f(): pass
> def test_foo(f, s):
>     pass
>
> s and f are unrelated and execute in (f, s) order in master, but in (s, f)
> order in my branch.
>
> Would like to hear what people think about this matter, as I think it is
> an important one, specially *Holger* if this is a design decision or just
> an accident of implementation, and if we should change it.
>
> Also, please feel free to just reply with what you thought what should be
> the behavior of the first sample.
>
> Cheers,
> Bruno.
> ​
>
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