nox and posargs where it starts with --

noxfile.py

import argparse

import nox

@nox.session(python=['2.7', '3.6'])
def tests(session):
    parser = argparse.ArgumentParser()
    parser.add_argument('--notest', action='store_true', default=False, help='Install dependencies. But do not run test.')
    parser.add_argument('args', nargs=argparse.REMAINDER)
    args = parser.parse_args(session.posargs)
    if not args.notest:
        session.run('pytest', *args.args)

"""
$ nox -r -- --foo
nox > Running session tests-2.7
nox > Re-using existing virtualenv at /tmp/.nox/tests-2-7.
usage: nox [-h] [--notest] ...
nox: error: unrecognized arguments: --foo

$ nox -r -- a --foo
nox > Running session tests-2.7
nox > Re-using existing virtualenv at /tmp/.nox/tests-2-7.
nox > pytest a --foo
...
"""