Add session.warn to output warnings (#482)

* Add `session.warn` to output warnings

Show warnings during the session.

* Update test_sessions.py

Add a test for "session.warn"
(It is pretty similar to "test_log", but it has changed the logging level to be tested).

* Update test_sessions.py

Run black to fix linting errors.

* Update test_sessions.py

Remove an additional whiespace.

nox/sessions.py

@@ -516,6 +516,10 @@ def log(self, *args: Any, **kwargs: Any) -> None:
  """Outputs a log during the session."""
  logger.info(*args, **kwargs)
 
+ def warn(self, *args: Any, **kwargs: Any) -> None:
+ """Outputs a warning during the session."""
+ logger.warning(*args, **kwargs)
+
  def error(self, *args: Any) -> "_typing.NoReturn":
  """Immediately aborts the session and optionally logs an error."""
  raise _SessionQuit(*args)

tests/test_sessions.py

@@ -601,6 +601,14 @@ def test_log(self, caplog):
 
  assert "meep" in caplog.text
 
+ def test_warn(self, caplog):
+ caplog.set_level(logging.WARNING)
+ session, _ = self.make_session_and_runner()
+
+ session.warn("meep")
+
+ assert "meep" in caplog.text
+
  def test_error(self, caplog):
  caplog.set_level(logging.ERROR)
  session, _ = self.make_session_and_runner()