Add a command line option for setting python=False

[sethmlarson]

How would this feature be useful?
If you have a noxfile.py that you use for development but also use Docker as a part of your CI you can skip having to create a virtualenv and reinstall dependencies that may have been installed as a part of your Docker image build step. For instance if I have:

FROM python:3

COPY dev-requirements.txt .
RUN python -m pip install -r dev-requirements.txt

COPY . .
RUN nox -s test --use-current-env

then my session.install("-r", "dev-requirements.txt") step within my noxfile.py would only require checking cache, much faster to run.

Describe the solution you'd like

Using --use-current-env (or alternative name, I'm fine with anything!) would override any python=[] within noxfile.py to python=False and use interpreter at sys.executable.

[theacodes]

I thought we already had this, but I'm not opposed to a venv backend that's a"passthrough".

[smarie]

There seem to be something like this already : from the doc

If you want to disable virtualenv creation altogether, you can set python to False

However you cannot use install nor conda_install then, you get ValueError: A session without a virtualenv can not install dependencies. or ValueError: A session without a conda environment can not install dependencies from conda..

I'll try to propose something to fix this

[smarie]

PR #316 provides a solution to this: you can now use --force-venv-backend (or -fb) to force a venv backend for all sessions to be run. Among supported backend, I added the "none" backend that is strictly equivalent to python=False.

So @sethmlarson you could do what you wish using nox -fb none. Of course we can imagine an even shorter option name such as --no-venv as a shortcut, actually I will try to submit it in the PR as it is much more readable. The advantage of --force-venv-backend (-fb) is that you can use it to switch to conda or venv too.

See #316 for details, there is also a new option to define the default venv backend.