Allow bypassing the install only flag

[s0undt3ch]

The reason behind this change is because, for our particular use case, we run some code against the virtualenv python in order to choose what requirements to install.
With this change, we can call nox with --install-only which skips all of the session.run calls, with the exception of those for which we explicitly pass bypass_install_only.

[dhermes]

This seems unnecessary. You can use session.posargs to pass through your option and then use a return statement in your session.

[dhermes]

You could also subclass Session and override install

[s0undt3ch]

You're missing the point. It has nothing to do with posargs.

Nox is responsible to install dependencies.
We lock dependencies based on python version and distro.
We need to know what's the python version on the virtualenv to install the right dependencies, and for that we get sys.version_info from the virtualenv's python, through a session.run.
If we can't know the version info, we can't install the right set of dependencies, hence, the proposed bypass.

[dhermes]

If you want to know the currently running Python:

@nox.session(py=("3.7", ...))
def foo(session):
    interpreter = session.virtualenv.interpreter
    ...

---

You can also scope requirements to specific Python versions with environment markers in a requirements.txt:

SomeProject == 5.4 ; python_version < '2.7'
OtherProject; sys_platform == 'win32'

---

Sorry for the misunderstanding. I was trying to say you could use posargs to achieve the same effect.

[s0undt3ch]

I'm not creating PRs just because...

I know I can get the python version from there, but our goal is to make our users life easier, so, we also have 2 and 3 as valid python versions(thus nox sessions too), but we lock our requirements against the python major AND minor version information.
So that won't work.

[s0undt3ch]

@nox.session(py=("2", ...))
def foo(session):
    interpreter = session.virtualenv.interpreter
    print(interpreter)
    ...

❯ nox -e 'runtests-zeromq-2(coverage=False)' -- -v --proxy
nox > Running session runtests-zeromq-2(coverage=False)
nox > Creating virtualenv using python2 in /home/vampas/projects/SaltStack/salt/branches/2017.7.9/.nox/runtests-zeromq-2-coverage-false
nox > Session runtests-zeromq-2(coverage=False) was successful.
nox > Running session runtests-parametrized-2(coverage=False, crypto=None, transport='zeromq')
nox > Creating virtualenv using python2 in /home/vampas/projects/SaltStack/salt/branches/2017.7.9/.nox/runtests-parametrized-2-coverage-false-crypto-none-transport-zeromq
2

notice the above lonely 2...

[theacodes]

Yeah this is too far. I don't like the idea of an argument that reverses a flag that itself reverses an argument.

It's just Python, you can use nox.command.run directly or use subprocess with session.virtualenv.bin if you need to here.

[s0undt3ch]

Your suggestions mean recreating the logic nox has on the noxfile or don't use nox at all(subprocess), that defeats the purpose of having nox to help out setting up the virtualenv...

[theacodes]

I think for now this still just doesn't appeal to my sensibilities. Another alternative is to use session arguments to do what you want:

@nox.session
def thing(session):
     if "setup-only" in session.posargs:
         ...
     else:
         ...

Or have a shared session like:

@nox.session
def setup(sesssion):
    ...

@nox.session
def thing(session)
    setup(session)
    ...

Happy to revisit this later as "no is temporary, but yes is forever" - https://twitter.com/solomonstre/status/715277134978113536?lang=en