nox.options.pythons ignored when nox.options.sessions is present

[cjolowicz]

Describe the bug

When both nox.options.sessions and nox.options.pythons are specified in noxfile.py, the latter setting 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 without options.

Actual behavior

• The tests session is run on Python 3.9 and 3.10.
• The launch_missiles session is not run at all.

nox > Running session tests-3.9
nox > Creating virtual environment (virtualenv) using python3.9 in .nox/tests-3-9
nox > Session tests-3.9 was successful.
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 > Ran multiple sessions:
nox > * tests-3.9: success
nox > * tests-3.10: success

Expected behavior

• The tests session is run on Python 3.9.
• The tests session is not run on Python 3.10.
• The launch_missiles session is not run at all.

nox > Running session tests-3.9
nox > Creating virtual environment (virtualenv) using python3.9 in .nox/tests-3-9
nox > Session tests-3.9 was successful.

Discussion

Before going into details, I'd like to thank you for creating and maintaining a wonderful tool! 💚

The function OptionSet.merge_namespaces merges options set in noxfile.py into the command-line options. Where applicable, this is done with the help of custom merge functions. The options --sessions, --keywords, and --pythons are handled 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.

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.

Unfortunately, OptionSet.merge_namespaces merges the command-line options in place. Merge functions such as _session_filters_merge_func inspect command_args to see if other options have been specified on the command-line. When the options are merged in place, this check produces false positives.

For example, nox.options.sessions is copied into command_args as a part of the merge; so it will appear to have been specified on the command-line when merging nox.options.pythons, causing the latter to be ignored.

Note that this is unrelated to the new --pythons option. For example, consider the following two nox files:

import nox

nox.options.sessions = ("tests",)
nox.options.keywords = "tests or launch_missiles"

@nox.session
def tests(session):
    pass

@nox.session
def launch_missiles(session):
    pass

import nox

nox.options.sessions = ("tests", "launch_missiles")
nox.options.keywords = "tests"

@nox.session
def tests(session):
    pass

@nox.session
def launch_missiles(session):
    pass

In the first version, sessions excludes launch_missiles, while keywords contains all sessions. When Nox is run without options, it will ignore keywords because sessions was already merged into the command-line options, and appears to have been specified on the command-line. As a result, only the tests session is run.

In the second version, sessions lists all sessions, while keywords excludes launch_missiles. When Nox is run without options, it will again ignore keywords because sessions was already merged into the command-line options. As a result, both tests and launch_missiles are run.

The order of precedence is determined by the declaration of options in nox._options: sessions -- pythons -- keywords.

Proposed solution

As a fix, I would propose that OptionSet.merge_namespaces copy command_args initially, and use that copy as the input for the merge functions, instead of the instance that is being mutated.