How would this feature be useful?

nox supports --no-install and -R options for not running installation commands, usually for use when reusing existing environments, and to speed up development where the install process is often the long pole.

But there doesn't seem to be a good way to indicate when something run via session.run is an install command -- specifically, if you're installing something not via pip or conda etc, which you may want to skip in the same scenarios as the above.

Concretely -- given a Javascript project, one might run (p)npm install to install all the project dependencies, followed by some test runner, e.g. jest, all under a noxenv called ui-tests.

In this scenario it would be nice if nox -s ui-tests -R was able to not run the install command, but the existing -R of course has no way to know which command to skip.

Even doing this externally (in the noxfile) seems nontrivial, as it seems the way session.install does this is by accessing private internals, e.g.:

Describe the solution you'd like

Add a session.run_install(...) command which runs a subprocess but assumes that subprocess is an installation command which will be skipped when using -R / --no-install.

Refactoring session.install and session.conda_install to call this to do their no_install check seems also like a nice minor internal refactor.

Describe alternatives you've considered

I personally don't like boolean flags (as a rule), but session.run(..., is_install=True) is an alternative for those who are less averse.

A further alternative is to make more things public API (e.g. whether no_install is active) so that someone can do this themselves in their noxfile, but given that nox itself has 2 places where it itself wants this, it seems like something generally useful to me.

Anything else?

A concrete noxfile where I wanted this functionality is here.