[pytest-dev] solving the "too static" fixture scope problem

[Florian Schulze]

I would expect the behaviour you describe for scope="each" to be the default for a fixture without a scope. Why introduce a new scope for that? Am I missing something? I haven't used pytest for such complex things yet.

Regarding the backward compatibility issue you described, I would expect the new behaviour instead of the current one. But maybe I'm missing something, because of the abstract example.

Regards,
Florian Schulze

On 11.10.2013, at 10:40, holger krekel <holger at merlinux.eu> wrote:

> Hi pytest users and developers,
> 
> I'd like to discuss and get feedback on 
> extending the fixture mechanism and fix what
> i consider a biggest limitation at the moment.
> 
> Problem: fixtures are tied statically to a scope
> =================================================
> 
> For example, you cannot use monkeypatch in a higher
> than "function" scoped fixture. Same is true for
> tmpdir and probably also many user-defined fixtures.
> I've certainly had this problem myself many times
> that i had a fixture function that didn't really
> care in what scope it was used.  There are
> ways to get around this problem but they are not
> pretty:
> 
>    @pytest.fixture(scope="module")
>    def myfix_module(request
>        return _myfix(request)
> 
>    @pytest.fixture(scope="function")
>    def myfix_function(request
>        return _myfix(request)
> 
> where _myfix is the function that doesn't
> care about the actual scope.  Even then, you
> can't use builtin fixtures like "monkeypatch",
> "tmpdir", etc.
> 
> Solution Idea: introduce "each" scoped fixtures
> =====================================================
> 
> The idea is allow a fixture function to declare it wants
> to be used in the scope of the requesting fixture function
> (or at function-scope if used from a test).
> 
> This is how "monkeypatch" would be implemented then:
> 
>    @pytest.fixture(scope="each")
>    def monkeypatch(request):
>        ...  # same implementation as in _pytest/monkeypatch.py
> 
> The new "each" scope means that each fixture/test requesting
> the "monkeypatch" fixture would receive its own fixture instance.
> 
> So a session-scoped fixture could naturally use it like this:
> 
>    @pytest.fixture(scope="session")
>    def myfix(monkeypatch):
>        monkeypatch.setattr(...)
>        return some_value
> 
> The passed in monkeypatch object here is a specific instance just 
> for the ``myfix`` fixture function: "each" fixture function
> requesting ``monkeypatch`` gets a new instance of it.  
> If e.g. a test uses another module-scoped fixture defined like this:
> 
>    @pytest.fixture(scope="module")
>    def myfix2(monkeypatch):
>        mp.setattr(...)
>        return some_value
> 
> this would invoke the ``monkeypatch`` fixture function a second time, 
> resulting in a new instance for use by the ``myfix2`` instance.
> 
> The same logic could be applied to other fixtures
> like "tmpdir" or user-defined ones.
> 
> Do you like this idea? Would you find it helpful for your test suites?
> 
> There is one issue i am not sure about yet, however.  Currently,
> when a test requires fixture A and B, and B requires C and C requires A,
> then the two "A" would be exactly the same object, independently of what
> which scopes are declared.  If A=="tmpdir", then the test's tmpdir and 
> C's tmpdir would be the same directory.  I often don't find this desirable.  
> If tmpdir would be an "each" scoped fixture, then C and the test would 
> each receive a clean new tmpdir.  If that is a backward-compat issue, 
> we could introduce another name for the new "each" scoped tmpdir.
> I usually find myself working around the problem of a "tmpdir"
> shared by multiple different fixtures, though.
> 
> cheers,
> holger
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