394 integrates no install flag

nox/_options.py

@@ -402,6 +402,14 @@ def _session_completer(
         hidden=True,
         finalizer_func=_color_finalizer,
     ),
+    _option_set.Option(
+        "no_install",
+        "--no-install",
+        default=False,
+        group=options.groups["secondary"],
+        action="store_true",
+        help="Skip invocations of session methods for installing packages.",
+    ),
 )
 
 

nox/sessions.py

@@ -290,6 +290,9 @@ def run_always(
             do not have a virtualenv.
         :type external: bool
         """
+        if self._runner.global_config.no_install:
+            return None
+
         if not args:
             raise ValueError("At least one argument required to run_always().")
 
@@ -368,6 +371,9 @@ def conda_install(
         if not args:
             raise ValueError("At least one argument required to install().")
 
+        if self._runner.global_config.no_install and venv._reused:
+            return None
+
         # Escape args that should be (conda-specific; pip install does not need this)
         args = _dblquote_pkg_install_args(args)
 
@@ -417,15 +423,20 @@ def install(self, *args: str, **kwargs: Any) -> None:
 
         .. _pip: https://pip.readthedocs.org
         """
+        venv = self._runner.venv
+
         if not isinstance(
-            self._runner.venv, (CondaEnv, VirtualEnv, PassthroughEnv)
+            venv, (CondaEnv, VirtualEnv, PassthroughEnv)
         ):  # pragma: no cover
             raise ValueError(
                 "A session without a virtualenv can not install dependencies."
             )
         if not args:
             raise ValueError("At least one argument required to install().")
 
+        if self._runner.global_config.no_install and venv._reused:
+            return None
+
         if "silent" not in kwargs:
             kwargs["silent"] = True
 

nox/virtualenv.py

@@ -54,6 +54,7 @@ class ProcessEnv:
     def __init__(self, bin_paths: None = None, env: Mapping[str, str] = None) -> None:
         self._bin_paths = bin_paths
         self.env = os.environ.copy()
+        self._reused = False
 
         if env is not None:
             self.env.update(env)
@@ -224,6 +225,9 @@ def create(self) -> bool:
             logger.debug(
                 "Re-using existing conda env at {}.".format(self.location_name)
             )
+
+            self._reused = True
+
             return False
 
         cmd = ["conda", "create", "--yes", "--prefix", self.location]
@@ -400,6 +404,7 @@ def create(self) -> bool:
                 [self._resolved_interpreter, "-c", "import sys; print(sys.prefix)"],
                 silent=True,
             )
+
             created = nox.command.run(
                 [
                     os.path.join(self.location, "bin", "python"),
@@ -414,6 +419,9 @@ def create(self) -> bool:
                         self.location_name
                     )
                 )
+
+                self._reused = True
+
                 return False
 
         if self.venv_or_virtualenv == "virtualenv":

tests/test_sessions.py

@@ -58,6 +58,17 @@ def test__normalize_path_give_up():
     assert "any-path" in norm_path
 
 
+class MockableSession(nox.sessions.Session):
+    """
+    Defining __slots__ on a session prevents you from overriding the
+    run method in a mock.
+    Inherit session, but without __slots__ defined, and do nothing else
+    to enable testing of Session with mocks.
+    """
+
+    pass
+
+
 class TestSession:
     def make_session_and_runner(self):
         func = mock.Mock(spec=["python"], python="3.7")
@@ -310,6 +321,19 @@ def test_run_always_install_only(self, caplog):
 
         assert session.run_always(operator.add, 23, 19) == 42
 
+    def test_run_always_no_install(self, caplog):
+        caplog.clear()
+        caplog.set_level(logging.INFO)
+
+        session, runner = self.make_session_and_runner()
+        runner.global_config.no_install = True
+
+        with mock.patch.object(nox.command, "run") as run:
+            assert session.run_always(operator.add, 23, 19) is None
+            run.assert_not_called()
+
+            assert "Skipping run_always" in caplog.text
+
     def test_conda_install_bad_args(self):
         session, runner = self.make_session_and_runner()
         runner.venv = mock.create_autospec(nox.virtualenv.CondaEnv)
@@ -359,10 +383,7 @@ def test_conda_install(self, auto_offline, offline):
         runner.venv.env = {}
         runner.venv.is_offline = lambda: offline
 
-        class SessionNoSlots(nox.sessions.Session):
-            pass
-
-        session = SessionNoSlots(runner=runner)
+        session = MockableSession(runner=runner)
 
         with mock.patch.object(session, "_run", autospec=True) as run:
             args = ("--offline",) if auto_offline and offline else ()
@@ -380,6 +401,23 @@ class SessionNoSlots(nox.sessions.Session):
                 external="error",
             )
 
+    def test_conda_venv_reused_with_no_install(self, caplog):
+        caplog.set_level(logging.INFO)
+        session, runner = self.make_session_and_runner()
+
+        runner.venv = mock.create_autospec(nox.virtualenv.CondaEnv)
+        runner.venv.location = "/path/to/conda/env"
+        runner.venv.env = {}
+        runner.venv.is_offline = lambda: True
+
+        runner.global_config.no_install = True
+        runner.venv._reused = True
+
+        with mock.patch.object(nox.command, "run"):
+            assert session.conda_install("baked beans", "eggs", "spam") is None
+
+        assert "Venv exists: skipping" in caplog.text
+
     @pytest.mark.parametrize(
         "version_constraint",
         ["no", "yes", "already_dbl_quoted"],
@@ -398,10 +436,7 @@ def test_conda_install_non_default_kwargs(self, version_constraint):
         runner.venv.env = {}
         runner.venv.is_offline = lambda: False
 
-        class SessionNoSlots(nox.sessions.Session):
-            pass
-
-        session = SessionNoSlots(runner=runner)
+        session = MockableSession(runner=runner)
 
         if version_constraint == "no":
             pkg_requirement = passed_arg = "urllib3"
@@ -453,10 +488,7 @@ def test_install(self):
         runner.venv = mock.create_autospec(nox.virtualenv.VirtualEnv)
         runner.venv.env = {}
 
-        class SessionNoSlots(nox.sessions.Session):
-            pass
-
-        session = SessionNoSlots(runner=runner)
+        session = MockableSession(runner=runner)
 
         with mock.patch.object(session, "_run", autospec=True) as run:
             session.install("requests", "urllib3")
@@ -482,10 +514,7 @@ def test_install_non_default_kwargs(self):
         runner.venv = mock.create_autospec(nox.virtualenv.VirtualEnv)
         runner.venv.env = {}
 
-        class SessionNoSlots(nox.sessions.Session):
-            pass
-
-        session = SessionNoSlots(runner=runner)
+        session = MockableSession(runner=runner)
 
         with mock.patch.object(session, "_run", autospec=True) as run:
             session.install("requests", "urllib3", silent=False)
@@ -538,6 +567,18 @@ def test_skip_no_log(self):
         with pytest.raises(nox.sessions._SessionSkip):
             session.skip()
 
+    def test_session_venv_reused_with_no_install(self, caplog):
+        caplog.clear()
+        caplog.set_level(logging.INFO)
+        session, runner = self.make_session_and_runner()
+        runner.global_config.no_install = True
+        runner.venv._reused = True
+
+        with mock.patch.object(nox.command, "run"):
+            assert session.install("eggs", "spam") is None
+
+        assert "Venv exists: skipping" in caplog.text
+
     def test___slots__(self):
         session, _ = self.make_session_and_runner()
         with pytest.raises(AttributeError):

tests/test_virtualenv.py

@@ -179,6 +179,7 @@ def test_condaenv_create(make_conda):
     venv.reuse_existing = True
     venv.create()
     assert dir_.join("test.txt").check()
+    assert venv._reused
 
 
 @pytest.mark.skipif(not HAS_CONDA, reason="Missing conda command.")
@@ -328,7 +329,10 @@ def test_create(monkeypatch, make_one):
     assert dir_.join("test.txt").check()
     venv.reuse_existing = True
     monkeypatch.setattr(nox.virtualenv.nox.command, "run", mock.MagicMock())
+
     venv.create()
+
+    assert venv._reused
     assert dir_.join("test.txt").check()