feat: support setting an empty session list

docs/config.rst

@@ -418,7 +418,7 @@ Or, if you wanted to provide a set of sessions that are run by default:
 The following options can be specified in the Noxfile:
 
 * ``nox.options.envdir`` is equivalent to specifying :ref:`--envdir <opt-envdir>`.
-* ``nox.options.sessions`` is equivalent to specifying :ref:`-s or --sessions <opt-sessions-pythons-and-keywords>`.
+* ``nox.options.sessions`` is equivalent to specifying :ref:`-s or --sessions <opt-sessions-pythons-and-keywords>`. If set to an empty list, no sessions will be run if no sessions were given on the commend line, and the list of available sessions will be shown instead.
 * ``nox.options.pythons`` is equivalent to specifying :ref:`-p or --pythons <opt-sessions-pythons-and-keywords>`.
 * ``nox.options.keywords`` is equivalent to specifying :ref:`-k or --keywords <opt-sessions-pythons-and-keywords>`.
 * ``nox.options.default_venv_backend`` is equivalent to specifying :ref:`-db or --default-venv-backend <opt-default-venv-backend>`.

nox/tasks.py

@@ -179,6 +179,10 @@ def filter_manifest(
             logger.error("Error while collecting sessions.")
             logger.error(exc.args[0])
             return 3
+    elif global_config.sessions is not None:
+        # If the user did not specify sessions, and the noxfile is set
+        # to an empty list, then list the sessions instead.
+        global_config.list_sessions = True
 
     # Filter by python interpreter versions.
     # This function never errors, but may cause an empty list of sessions

tests/test_tasks.py

@@ -346,6 +346,20 @@ def test_honor_list_request_doesnt_print_docstring_if_not_present(capsys):
     assert "Hello I'm a docstring" not in out
 
 
+def test_emtpy_session_list_in_noxfile():
+    config = _options.options.namespace(noxfile="noxfile.py", sessions=())
+    manifest = Manifest({}, config)
+    tasks.filter_manifest(manifest, global_config=config)
+    assert config.list_sessions
+
+
+def test_emtpy_session_None_in_noxfile():
+    config = _options.options.namespace(noxfile="noxfile.py", sessions=None)
+    manifest = Manifest({}, config)
+    tasks.filter_manifest(manifest, global_config=config)
+    assert not config.list_sessions
+
+
 def test_verify_manifest_empty():
     config = _options.options.namespace(sessions=(), keywords=())
     manifest = Manifest({}, config)