Missing interpreters now error the session on CI by default

docs/usage.rst

@@ -218,14 +218,16 @@ If the Noxfile sets ``nox.options.stop_on_first_error``, you can override the No
 Failing sessions when the interpreter is missing
 ------------------------------------------------
 
-By default, Nox will skip sessions where the Python interpreter can't be found. If you want Nox to mark these sessions as failed, you can use ``--error-on-missing-interpreters``:
+By default, when not on CI, Nox will skip sessions where the Python interpreter can't be found. If you want Nox to mark these sessions as failed, you can use ``--error-on-missing-interpreters``:
 
 .. code-block:: console
 
     nox --error-on-missing-interpreters
 
 If the Noxfile sets ``nox.options.error_on_missing_interpreters``, you can override the Noxfile setting from the command line by using ``--no-error-on-missing-interpreters``.
 
+If being run on Continuous Integration (CI) systems, Nox will treat missing interpreters as errors by default to avoid sessions silently passing when the requested python interpreter is not installed. Nox does this by looking for an environment variable called ``CI`` which is a convention used by most CI providers.
+
 .. _opt-error-on-external-run:
 
 Disallowing external programs

nox/_option_set.py

@@ -157,6 +157,7 @@ def make_flag_pair(
     name: str,
     enable_flags: tuple[str, str] | tuple[str],
     disable_flags: tuple[str],
+    default: bool = False,
     **kwargs: Any,
 ) -> tuple[Option, Option]:
     """Returns two options - one to enable a behavior and another to disable it.
@@ -172,6 +173,7 @@ def make_flag_pair(
         *enable_flags,
         noxfile=True,
         merge_func=functools.partial(flag_pair_merge_func, name, disable_name),
+        default=default,
         **kwargs,
     )
 

nox/_options.py

@@ -419,6 +419,7 @@ def _session_completer(
         ("--no-error-on-missing-interpreters",),
         group=options.groups["execution"],
         help="Error instead of skipping sessions if an interpreter can not be located.",
+        default=True if "CI" in os.environ else False,
     ),
     *_option_set.make_flag_pair(
         "error_on_external_run",

nox/sessions.py

@@ -694,6 +694,9 @@ def execute(self) -> Result:
             if self.global_config.error_on_missing_interpreters:
                 return Result(self, Status.FAILED, reason=str(exc))
             else:
+                logger.warning(
+                    "Missing interpreters will error by default on CI systems."
+                )
                 return Result(self, Status.SKIPPED, reason=str(exc))
 
         except _SessionQuit as exc:

tests/test_sessions.py

@@ -749,7 +749,7 @@ def make_runner(self):
                 envdir="envdir",
                 posargs=[],
                 reuse_existing_virtualenvs=False,
-                error_on_missing_interpreters=False,
+                error_on_missing_interpreters=True if "CI" in os.environ else False,
             ),
             manifest=mock.create_autospec(nox.manifest.Manifest),
         )
@@ -904,14 +904,31 @@ def test_execute_with_manifest_null_session_func(self):
         assert result.status == nox.sessions.Status.SKIPPED
         assert "no parameters" in result.reason
 
-    def test_execute_skip_missing_interpreter(self):
+    def test_execute_skip_missing_interpreter(self, caplog, monkeypatch):
+        # Important to have this first here as the runner will pick it up
+        # to set default for --error-on-missing-interpreters
+        monkeypatch.delenv("CI", raising=False)
+
         runner = self.make_runner_with_mock_venv()
         runner._create_venv.side_effect = nox.virtualenv.InterpreterNotFound("meep")
 
         result = runner.execute()
 
         assert result.status == nox.sessions.Status.SKIPPED
         assert "meep" in result.reason
+        assert (
+            "Missing interpreters will error by default on CI systems." in caplog.text
+        )
+
+    def test_execute_missing_interpreter_on_CI(self, monkeypatch):
+        monkeypatch.setenv("CI", "True")
+        runner = self.make_runner_with_mock_venv()
+        runner._create_venv.side_effect = nox.virtualenv.InterpreterNotFound("meep")
+
+        result = runner.execute()
+
+        assert result.status == nox.sessions.Status.FAILED
+        assert "meep" in result.reason
 
     def test_execute_error_missing_interpreter(self):
         runner = self.make_runner_with_mock_venv()