fix: error if invalid reuse_venv set (#872)

Signed-off-by: Henry Schreiner <[email protected]>

nox/sessions.py

@@ -1053,6 +1053,12 @@ def reuse_existing_venv(self) -> bool:
         Returns:
             bool: True if the existing virtual environment should be reused, False otherwise.
         """
+        if self.global_config.reuse_venv not in {"always", "never", "no", "yes", None}:
+            msg = (
+                "nox.options.reuse_venv must be set to 'always', 'never', 'no', or 'yes',"
+                f" got {self.global_config.reuse_venv!r}!"
+            )
+            raise AttributeError(msg)
 
         return any(
             (

tests/test_sessions.py

@@ -18,6 +18,7 @@
 import logging
 import operator
 import os
+import re
 import shutil
 import subprocess
 import sys
@@ -1145,6 +1146,13 @@ def test__reuse_venv_outcome(self, reuse_venv, reuse_venv_func, should_reuse):
         runner.global_config.reuse_venv = reuse_venv
         assert runner.reuse_existing_venv() == should_reuse
 
+    def test__reuse_venv_invalid(self):
+        runner = self.make_runner()
+        runner.global_config.reuse_venv = True
+        msg = "nox.options.reuse_venv must be set to 'always', 'never', 'no', or 'yes', got True!"
+        with pytest.raises(AttributeError, match=re.escape(msg)):
+            runner.reuse_existing_venv()
+
     def make_runner_with_mock_venv(self):
         runner = self.make_runner()
         runner._create_venv = mock.Mock()