Check for NOXSESSION environment variable

docs/usage.rst

@@ -56,6 +56,11 @@ By default nox will run all sessions defined in the noxfile. However, you can ch
     nox -s lint tests
     nox -e lint
 
+You can also use the ``NOXSESSION`` environment variable::
+
+    NOXSESSION=lint nox
+    NOXSESSION=lint,tests nox
+
 Nox will run these sessions in the same order they are specified.
 
 You can also use `pytest-style keywords`_ to filter test sessions::

nox/__main__.py

@@ -68,6 +68,7 @@ def main():
         "-e",
         "--sessions",
         nargs="*",
+        default=_get_default_sessions(),
         help="Which sessions to run, by default, all sessions will run.",
     )
     parser.add_argument(
@@ -136,5 +137,11 @@ def main():
     sys.exit(exit_code)
 
 
+def _get_default_sessions():
+    nox_env = os.environ.get("NOXSESSION")
+    env_sessions = nox_env.split(",") if nox_env else None
+    return env_sessions
+
+
 if __name__ == "__main__":  # pragma: no cover
     main()

tests/test_main.py

@@ -19,6 +19,7 @@
 
 import contexter
 import pkg_resources
+import pytest
 
 import nox
 import nox.__main__
@@ -59,7 +60,9 @@ def test_global_config_constructor():
     assert config.posargs == ["a", "b", "c"]
 
 
-def test_main_no_args():
+def test_main_no_args(monkeypatch):
+    # Prevents any interference from outside
+    monkeypatch.delenv("NOXSESSION", raising=False)
     sys.argv = [sys.executable]
     with mock.patch("nox.workflow.execute") as execute:
         execute.return_value = 0
@@ -146,6 +149,28 @@ def test_main_explicit_sessions():
         assert config.sessions == ["1", "2"]
 
 
+@pytest.mark.parametrize(
+    "env,sessions", [("foo", ["foo"]), ("foo,bar", ["foo", "bar"])]
+)
+def test_main_session_from_nox_env_var(monkeypatch, env, sessions):
+    monkeypatch.setenv("NOXSESSION", env)
+    sys.argv = [sys.executable]
+    with mock.patch("nox.workflow.execute") as execute:
+        execute.return_value = 0
+
+        # Call the main function.
+        with mock.patch.object(sys, "exit") as exit:
+            nox.__main__.main()
+            exit.assert_called_once_with(0)
+        assert execute.called
+
+        # Verify that the sessions from the env var are listed in the config.
+        config = execute.call_args[1]["global_config"]
+        assert len(config.sessions) == len(sessions)
+        for session in sessions:
+            assert session in config.sessions
+
+
 def test_main_positional_args():
     sys.argv = [sys.executable, "1", "2", "3"]
     with mock.patch("nox.workflow.execute") as execute: