Disable running session.install outside a venv (#580)

* Disable running `session.install` outside a venv

* Fix linting

* Replace the old test by a new one (almost)

Co-authored-by: Claudio Jolowicz <mail@claudiojolowicz.com>

nox/sessions.py

@@ -22,7 +22,6 @@
 import re
 import sys
 import unicodedata
-import warnings
 from types import TracebackType
 from typing import Any, Callable, Iterable, Mapping, Sequence
 
@@ -533,6 +532,12 @@ def install(self, *args: str, **kwargs: Any) -> None:
 
         Additional keyword args are the same as for :meth:`run`.
 
+        .. warning::
+
+            Running ``session.install`` without a virtual environment
+            is no longer supported. If you still want to do that, please
+            use ``session.run("pip", "install", ...)`` instead.
+
         .. _pip: https://pip.readthedocs.org
         """
         venv = self._runner.venv
@@ -544,13 +549,11 @@ def install(self, *args: str, **kwargs: Any) -> None:
                 "A session without a virtualenv can not install dependencies."
             )
         if isinstance(venv, PassthroughEnv):
-            warnings.warn(
-                f"Session {self.name} does not have a virtual environment, "
-                "so use of session.install() is deprecated since it would modify "
-                "the global Python environment. If you're really sure that is "
-                'what you want to do, use session.run("pip", "install", ...) instead.',
-                category=FutureWarning,
-                stacklevel=2,
+            raise ValueError(
+                f"Session {self.name} does not have a virtual environment, so use of"
+                " session.install() is no longer allowed since it would modify the"
+                " global Python environment. If you're really sure that is what you"
+                ' want to do, use session.run("pip", "install", ...) instead.'
             )
         if not args:
             raise ValueError("At least one argument required to install().")

tests/test_sessions.py

@@ -646,7 +646,7 @@ class SessionNoSlots(nox.sessions.Session):
                 external="error",
             )
 
-    def test_install_no_venv_deprecated(self):
+    def test_install_no_venv_failure(self):
         runner = nox.sessions.SessionRunner(
             name="test",
             signatures=["test"],
@@ -662,25 +662,14 @@ class SessionNoSlots(nox.sessions.Session):
 
         session = SessionNoSlots(runner=runner)
 
-        with mock.patch.object(session, "_run", autospec=True) as run:
-            with pytest.warns(
-                FutureWarning,
-                match=(
-                    r"use of session\.install\(\) is deprecated since it would modify"
-                    r" the global Python environment"
-                ),
-            ):
-                session.install("requests", "urllib3")
-            run.assert_called_once_with(
-                "python",
-                "-m",
-                "pip",
-                "install",
-                "requests",
-                "urllib3",
-                silent=True,
-                external="error",
-            )
+        with pytest.raises(
+            ValueError,
+            match=(
+                r"use of session\.install\(\) is no longer allowed since"
+                r" it would modify the global Python environment"
+            ),
+        ):
+            session.install("requests", "urllib3")
 
     def test_notify(self):
         session, runner = self.make_session_and_runner()