pytest 3.3.0 immutable fixture params

[popravich]

Before v3.3.0 it was possible to mutate fixture parameters, since v3.3.0 params gets "frozen"
(I guess its introduced with this commit 07b2b18)

I was using this feature to setup fixture parameters based on command-line options:

BINS = []

@pytest.fixture(scope='session', params=BINS, ids=...)
def server_bin(request):
    return request.param

@pytest.fixture(scope='session')
def run_server(server_bin):
    pass # start application

def pytest_configure(config):
    BINS[:] = config.getoption('--server')[:]

• Here is the real use case https://github.com/aio-libs/aioredis/blob/pyup/tests/conftest.py#L245-L254
• Here are the travis builds before and after update;

So my questions:

• Is this new behavior intended or its a bug?
• And if its intended what is new way to make dynamic parameters?

[popravich]

The same case is here https://github.com/aio-libs/aiohttp/blob/master/aiohttp/pytest_plugin.py#L195-L200

[RonnyPfannschmidt]

its neither intended, not was it a bug

we simply didn't fall over because users would mutate random aliased objects that would then end up in the internals

the correct way to make fixtures dynamic is something like this:

def pytest_configure(config):
  class DynamicFixturePlugin(object):
     @pytest.fixture(scope='session', params=config.getoption('--servers'), ids=...)
     def server_bin(request):
        return request.param
    config.pluginmanager.register(DynamicFixturePlugin(), 'server-bin-fixture')

[popravich]

Ok, thanks,
I will try it

[nicoddemus]

Do we still consider this regression?

[RonnyPfannschmidt]

@nicoddemus it practically is a regression, but its one i'd like to not fix since fixing it makes things worse in the internals again

[nicoddemus]

@RonnyPfannschmidt I agree, doesn't seem like something we want to actually fix, at least not in its current form where it was working by accident rather than by design.

@popravich when you can please let us know if the workaround worked for you.

[argaen]

I just found out about this today. Is it worth it to at least add a disclaimer somehow in the CHANGELOG? Our deployment to PROD was skipping some tests without us even noticing so I believe its something worth to comment.

[popravich]

@nicoddemus, the suggested code works with a tiny fix

def pytest_configure(config):
  class DynamicFixturePlugin(object):
     @pytest.fixture(scope='session', params=config.getoption('--servers'), ids=...)
-     def server_bin(request):
+     def server_bin(self, request):
        return request.param
    config.pluginmanager.register(DynamicFixturePlugin(), 'server-bin-fixture')

And I'm +1 to @argaen about some notes in CHANGELOG.

[nicoddemus]

@argaen and @popravich agree, thanks for the suggestion: #2980