Document how runs cause sessions to fail

[ktbarrett]

How would this feature be useful?

I need to always run cleanup code when a task fails. I didn't know how to accomplish that until I realized that sessions fail when session.runs raise CommandFailed. The use of exceptions here means I can cleanup by using a finally block. For example,

@nox.session(reuse_venv=True)
def check(session):
    build_env(session)
    try:
        session.run("git", "stash", "-k", "-u")
        session.run("isort", "--check-only", "--profile=black", ".")
        session.run("black", "--check", ".")
        session.run("mypy", "--strict", ".")
    finally:
        session.run("git", "stash", "pop")

Describe the solution you'd like

Taking the outcome of #516 into account, documenting that this is possible may help other users learn understand what is possible.

[cjolowicz]

Thanks for catching this! Indeed, the documentation is not explicit about what happens when a command run via session.run fails. This could be a sentence added to the docstring of nox.sessions.Session.run.

I also agree that the pattern of wrapping a try...finally around session.run for cleanup actions is worth mentioning. Not sure about mentioning CommandFailed specifically, as it's currently not part of the public API. Maybe for now it's enough to just say that an exception is raised that causes Nox to mark the session as failed?

[ktbarrett]

Also, maybe I missed it, but I never saw it mentioned anywhere that session.run fails the current session immediately.

session.run causes the current session to fail immediately
all sessions listed are always run
if any session fails, the nox invocation fails