Merge pull request #9911 from bkeyvani/fix-issue-8646

Author: Zac-HD

AUTHORS

@@ -44,6 +44,7 @@ Aron Coyle
 Aron Curzon
 Aviral Verma
 Aviv Palivoda
+Babak Keyvani
 Barney Gale
 Ben Gartner
 Ben Webb

changelog/8646.improvement.rst

@@ -0,0 +1,2 @@
+Improve :py:func:`pytest.raises`. Previously passing an empty tuple would give a confusing
+error. We now raise immediately with a more helpful message.

src/_pytest/python_api.py

@@ -899,6 +899,12 @@ def raises(
     """
     __tracebackhide__ = True
 
+    if not expected_exception:
+        raise ValueError(
+            f"Expected an exception type or a tuple of exception types, but got `{expected_exception!r}`. "
+            f"Raising exceptions is already understood as failing the test, so you don't need "
+            f"any special code to say 'this should never raise an exception'."
+        )
     if isinstance(expected_exception, type):
         excepted_exceptions: Tuple[Type[E], ...] = (expected_exception,)
     else:

testing/python/raises.py

@@ -19,6 +19,16 @@ def test_raises_function(self):
         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"):
+            # We're testing that this invalid usage gives a helpful error,
+            # so we can ignore Mypy telling us that None is invalid.
+            pytest.raises(expected_exception=None)  # type: ignore
+
+    def test_raises_does_not_allow_empty_tuple(self):
+        with pytest.raises(ValueError, match="Expected an exception type or"):
+            pytest.raises(expected_exception=())
+
     def test_raises_callable_no_exception(self) -> None:
         class A:
             def __call__(self):