nox.options.sessions ignored when --pythons is passed

[cjolowicz]

Describe the bug

When nox.options.sessions is specified in noxfile.py and --pythons is passed on the command-line, the setting in the noxfile.py is ignored.

How to reproduce

import nox

nox.options.sessions = ("tests",)
nox.options.pythons = ("3.9",)

@nox.session(python=("3.9", "3.10"))
def tests(session):
    pass

@nox.session(python=("3.9", "3.10"))
def launch_missiles(session):
    pass

Run nox --python=3.10.

Actual behavior

• The tests session is run on Python 3.10.
• The launch_missiles session is run on Python 3.10.

nox > Running session tests-3.10
nox > Creating virtual environment (virtualenv) using python3 in .nox/tests-3-10
nox > Session tests-3.10 was successful.
nox > Running session launch_missiles-3.10
nox > Creating virtual environment (virtualenv) using python3 in .nox/launch_missiles-3-10
nox > Session launch_missiles-3.10 was successful.
nox > Ran multiple sessions:
nox > * tests-3.10: success
nox > * launch_missiles-3.10: success

Expected behavior

• The tests session is run on Python 3.10.
• The launch_missiles session is not run at all.

nox > Running session tests-3.10
nox > Creating virtual environment (virtualenv) using python3 in .nox/tests-3-10
nox > Session tests-3.10 was successful.

Discussion

This is a follow-up to #356, which has an associated pull request in #357.

When --pythons was introduced, the merge logic for --sessions and --keywords was extended to the new --pythons option. This means that if any of these options are specified on the command line, the corresponding settings in the noxfile.py are ignored. This is achieved by a custom merge function session_filters_merge_func, which ensures that noxfile.py is ignored for all of these options if any of them are set on the command-line.

In the case of --sessions and --keywords, this makes sense, because these options are different means to achieve the same goal of specifying which sessions should run. For example, if noxfile.py contains nox.options.keywords = "not launch_missiles", we want the user to still be able to trigger the session explicitly using --session=launch_missiles; the custom merge function achieves that. Without the custom merge function, the user would be forced to use --keywords instead of --sessions.

In the case of --pythons, I would argue that this behavior is less useful. While it is true that the --pythons option, just like the other two options, ultimately results in filtering the session manifest, the new option is particularly useful in combination with the other options, not as a way to override them. Unlike --sessions and --keywords, it is not possible to use --pythons to completely control which sessions are selected. Therefore it makes less sense to me to completely ignore the other options when --pythons is passed.

I would also argue that the current behavior violates the principle of least surprise. The example above demonstrates this: While the noxfile.py clearly states the intent that launch_missiles should only run on explicit user request, it is enough to specify a Python version on the command line to trigger the session, without specifying the session itself.

In my opinion, the merge logic for --pythons should be decoupled from that of --sessions and --keywords. This would change current behavior in two ways:

• The nox.options.sessions and nox.options.keywords settings would no longer be ignored when --pythons is passed on the command-line.
• The nox.options.pythons would no longer be ignored when --sessions or --keywords are passed on the command-line.

(The second point will make it possible to run specific sessions using -s or -k without having to repeat nox.options.pythons on the command-line.)

Proposed solution

As a fix, I would propose the following two changes, in addition to the change proposed in #357:

1. Revert _session_filters_merge_func to the state before --pythons was introduced. This means that it will only check if --sessions or --keywords were specified on the command-line. As a result, the nox.options.sessions and nox.options.keywords will no longer be ignored when --pythons is passed.
2. Declare the --python option without a custom merge function, i.e. omit the merge_func parameter. As a result, nox.options.pythons will no longer be ignored when --sessions or --keywords are passed.