Add test fixture to generate noxfile.py

The noxfile.py is templated with the default session (`nox.options.sessions`),
and the default Python version (`nox.options.pythons`), as well as an alternate
Python version.

This allows us to avoid the situation where a test case running on one Python
version needs to launch Nox using another Python version. As a side-effect, it
makes the test cases a bit more explicit.

nox/command.py

@@ -14,7 +14,7 @@
 
 import os
 import sys
-from typing import Any, Iterable, Optional, Sequence, Union, List
+from typing import Any, Iterable, List, Optional, Sequence, Union
 
 import py
 from nox.logger import logger

nox/virtualenv.py

@@ -16,9 +16,9 @@
 import platform
 import re
 import shutil
-from socket import gethostbyname
 import sys
-from typing import Any, Mapping, Optional, Tuple, Union, List
+from socket import gethostbyname
+from typing import Any, List, Mapping, Optional, Tuple, Union
 
 import nox.command
 import py

tests/resources/noxfile_options_pythons.py

@@ -0,0 +1,28 @@
+# Copyright 2020 Alethea Katherine Flowers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import nox
+
+nox.options.sessions = ["{default_session}"]
+nox.options.pythons = ["{default_python}"]
+
+
+@nox.session(python=["{default_python}", "{alternate_python}"])
+def test(session):
+    pass
+
+
+@nox.session(python=["{default_python}", "{alternate_python}"])
+def launch_rocket(session):
+    pass

tests/test_logger.py

@@ -16,7 +16,6 @@
 from unittest import mock
 
 import pytest
-
 from nox import logger
 
 

tests/test_main.py

@@ -14,6 +14,7 @@
 
 import os
 import sys
+from pathlib import Path
 from unittest import mock
 
 import contexter
@@ -444,6 +445,30 @@ def test_main_noxfile_options_sessions(monkeypatch):
         assert config.sessions == ["test"]
 
 
+@pytest.fixture
+def generate_noxfile_options_pythons(tmp_path):
+    """Generate noxfile.py with test and launch_rocket sessions.
+
+    The sessions are defined for both the default and alternate Python versions.
+    The ``default_session`` and ``default_python`` parameters determine what
+    goes into ``nox.options.sessions`` and ``nox.options.pythons``, respectively.
+    """
+
+    def generate_noxfile(default_session, default_python, alternate_python):
+        path = Path(RESOURCES) / "noxfile_options_pythons.py"
+        text = path.read_text()
+        text = text.format(
+            default_session=default_session,
+            default_python=default_python,
+            alternate_python=alternate_python,
+        )
+        path = tmp_path / "noxfile.py"
+        path.write_text(text)
+        return str(path)
+
+    return generate_noxfile
+
+
 @pytest.mark.parametrize(("isatty_value", "expected"), [(True, True), (False, False)])
 def test_main_color_from_isatty(monkeypatch, isatty_value, expected):
     monkeypatch.setattr(sys, "argv", [sys.executable])