improve `pytest.raises` - cont'd

a few more iterations on error message and related tests.

src/_pytest/python_api.py

@@ -899,12 +899,11 @@ def raises(
     """
     __tracebackhide__ = True
 
-    if expected_exception == ():
+    if not expected_exception:
         raise ValueError(
-            "Passing expected_exception=() is an error, because it's impossible to "
+            f"Expected an exception type or a tuple of exception types, but got `{expected_exception!r}`. "
-            "raise an exception which is not an instance of any type. Raising exceptions "
+            f"Raising exceptions is already understood as failing the test, so you don't need "
-            "is already understood as failing the test, so you don't need any special "
+            f"any special code to say 'this should never raise an exception'."
-            "code to say 'this should never raise an exception'."
         )
     if isinstance(expected_exception, type):
         excepted_exceptions: Tuple[Type[E], ...] = (expected_exception,)

testing/python/raises.py

@@ -19,8 +19,12 @@ class TestRaises:
         excinfo = pytest.raises(ValueError, int, "hello")
         assert "invalid literal" in str(excinfo.value)
 
+    def test_raises_does_not_allow_none(self):
+        with pytest.raises(ValueError, match="Expected an exception type or"):
+            pytest.raises(expected_exception=None)
+
     def test_raises_does_not_allow_empty_tuple(self):
-        with pytest.raises(ValueError):
+        with pytest.raises(ValueError, match="Expected an exception type or"):
             pytest.raises(expected_exception=())
 
     def test_raises_callable_no_exception(self) -> None: