Skip conda_tests session by default if user does not have conda

[FollowTheProcess]

Fixes #520

Adds a simple session filter at the top of our noxfile.py that runs all sessions (other than conda_tests) by default, only adding conda_tests if the user has conda installed.

# Skip 'conda_tests' if user doesn't have conda installed
nox.options.sessions = ["tests", "cover", "blacken", "lint", "docs"]
if shutil.which("conda"):
    nox.options.sessions.append("conda_tests")

I also spotted a stray conda-dependent test without a pytest.mark.skipif(not HAS_CONDA) guard on it so I quickly added that too:

@pytest.mark.skipif(not HAS_CONDA, reason="Missing conda command.")
def test_condaenv_(make_conda):
    venv, dir_ = make_conda()
    assert not venv.is_offline()

[DiddiLeija]

This works! Thank you @FollowTheProcess!

[cjolowicz]

Thanks for taking care of this!

How about using session.skip?

def conda_tests(session):
    if not shutil.which("conda"):
        session.skip("conda is not installed")
    ...

Nox's output will then make it clear that the session was skipped and why.

[FollowTheProcess]

@cjolowicz Didn't know about session.skip, that should be perfect. Weirdly though I still get the error!

I'm guessing that the

nox.session(..., venv_backend=conda)

is what triggers it? In which case it probably never gets to the session.skip line?

[cjolowicz]

Oh right, that makes sense.