Derive pytest.raises from AbstractContextManager

Makes `AbstractContextManager` the shared base class between "raises" and other context managers.

The motivation is for type checkers to narrow `pytest.raises(...) if x else nullcontext()` to a `ContextManager` rather than `object`.

changelog/10660.bugfix.rst

@@ -0,0 +1,2 @@
+Fix :py:func:`pytest.raises` to return a 'ContextManager' so that type-checkers could narrow
+:code:`pytest.raises(...) if ... else nullcontext()` down to 'ContextManager' rather than 'object'.

src/_pytest/python_api.py

@@ -8,7 +8,7 @@ from types import TracebackType
 from typing import Any
 from typing import Callable
 from typing import cast
-from typing import Generic
+from typing import ContextManager
 from typing import List
 from typing import Mapping
 from typing import Optional
@@ -957,7 +957,7 @@ raises.Exception = fail.Exception  # type: ignore
 
 
 @final
-class RaisesContext(Generic[E]):
+class RaisesContext(ContextManager[_pytest._code.ExceptionInfo[E]]):
     def __init__(
         self,
         expected_exception: Union[Type[E], Tuple[Type[E], ...]],

testing/typing_checks.py

@@ -3,6 +3,11 @@
 This file is not executed, it is only checked by mypy to ensure that
 none of the code triggers any mypy errors.
 """
+import contextlib
+from typing import Optional
+
+from typing_extensions import assert_type
+
 import pytest
 
 
@@ -22,3 +27,9 @@ def check_fixture_ids_callable() -> None:
 @pytest.mark.parametrize("func", [str, int], ids=lambda x: str(x.__name__))
 def check_parametrize_ids_callable(func) -> None:
     pass
+
+
+def check_raises_is_a_context_manager(val: bool) -> None:
+    with pytest.raises(RuntimeError) if val else contextlib.nullcontext() as excinfo:
+        pass
+    assert_type(excinfo, Optional[pytest.ExceptionInfo[RuntimeError]])