fix: make --tags behave like --sessions and --keywords

nox/_options.py

@@ -66,11 +66,11 @@
 )
 
 
-def _sessions_and_keywords_merge_func(
+def _sessions_merge_func(
     key: str, command_args: argparse.Namespace, noxfile_args: argparse.Namespace
 ) -> list[str]:
-    """Only return the Noxfile value for sessions/keywords if neither sessions
-    or keywords are specified on the command-line.
+    """Only return the Noxfile value for sessions/keywords if neither sessions,
+    keywords or tags are specified on the command-line.
 
     Args:
         key (str): This function is used for both the "sessions" and "keywords"
@@ -80,7 +80,11 @@ def _sessions_and_keywords_merge_func(
             command-line.
         noxfile_Args (_option_set.Namespace): The options specified in the
             Noxfile."""
-    if not command_args.sessions and not command_args.keywords:
+    if (
+        not command_args.sessions
+        and not command_args.keywords
+        and not command_args.tags
+    ):
         return getattr(noxfile_args, key)  # type: ignore[no-any-return]
     return getattr(command_args, key)  # type: ignore[no-any-return]
 
@@ -280,7 +284,7 @@ def _session_completer(
         "--session",
         group=options.groups["sessions"],
         noxfile=True,
-        merge_func=functools.partial(_sessions_and_keywords_merge_func, "sessions"),
+        merge_func=functools.partial(_sessions_merge_func, "sessions"),
         nargs="*",
         default=default_env_var_list_factory("NOXSESSION"),
         help="Which sessions to run. By default, all sessions will run.",
@@ -303,7 +307,7 @@ def _session_completer(
         "--keywords",
         group=options.groups["sessions"],
         noxfile=True,
-        merge_func=functools.partial(_sessions_and_keywords_merge_func, "keywords"),
+        merge_func=functools.partial(_sessions_merge_func, "keywords"),
         help="Only run sessions that match the given expression.",
     ),
     _option_set.Option(
@@ -312,6 +316,7 @@ def _session_completer(
         "--tags",
         group=options.groups["sessions"],
         noxfile=True,
+        merge_func=functools.partial(_sessions_merge_func, "tags"),
         nargs="*",
         help="Only run sessions with the given tags.",
     ),

tests/test_tasks.py

@@ -315,6 +315,31 @@ def quux():
     assert "Tag selection caused no sessions to be selected." in caplog.text
 
 
+def test_merge_sessions_and_tags():
+    @nox.session(tags=["foobar"])
+    def test():
+        pass
+
+    @nox.session(tags=["foobar"])
+    def bar():
+        pass
+
+    config = _options.options.namespace(
+        noxfile=os.path.join(RESOURCES, "noxfile_options.py"),
+        sessions=None,
+        pythons=(),
+        posargs=[],
+        tags=["foobar"],
+    )
+
+    nox_module = tasks.load_nox_module(config)
+    tasks.merge_noxfile_options(nox_module, config)
+    manifest = Manifest({"test": test, "bar": bar}, config)
+    return_value = tasks.filter_manifest(manifest, config)
+    assert return_value is manifest
+    assert len(manifest) == 2
+
+
 def test_honor_list_request_noop():
     config = _options.options.namespace(list_sessions=False)
     manifest = {"thing": mock.sentinel.THING}