Make ExceptionInfo generic in the exception type

This way, in

    with pytest.raises(ValueError) as cm:
        ...

cm.value is a ValueError and not a BaseException.

Author: bluetech

src/_pytest/_code/code.py

@@ -6,9 +6,11 @@
 from inspect import CO_VARKEYWORDS
 from traceback import format_exception_only
 from types import TracebackType
+from typing import Generic
 from typing import Optional
 from typing import Pattern
 from typing import Tuple
+from typing import TypeVar
 from typing import Union
 from weakref import ref
 
@@ -379,22 +381,25 @@ def recursionindex(self):
 )
 
 
+_E = TypeVar("_E", bound=BaseException)
+
+
 @attr.s(repr=False)
-class ExceptionInfo:
+class ExceptionInfo(Generic[_E]):
     """ wraps sys.exc_info() objects and offers
         help for navigating the traceback.
     """
 
     _assert_start_repr = "AssertionError('assert "
 
-    _excinfo = attr.ib(
-        type=Optional[Tuple["Type[BaseException]", BaseException, TracebackType]]
-    )
+    _excinfo = attr.ib(type=Optional[Tuple["Type[_E]", "_E", TracebackType]])
     _striptext = attr.ib(type=str, default="")
     _traceback = attr.ib(type=Optional[Traceback], default=None)
 
     @classmethod
-    def from_current(cls, exprinfo: Optional[str] = None) -> "ExceptionInfo":
+    def from_current(
+        cls, exprinfo: Optional[str] = None
+    ) -> "ExceptionInfo[BaseException]":
         """returns an ExceptionInfo matching the current traceback
 
         .. warning::
@@ -422,21 +427,21 @@ def from_current(cls, exprinfo: Optional[str] = None) -> "ExceptionInfo":
         return cls(tup, _striptext)
 
     @classmethod
-    def for_later(cls) -> "ExceptionInfo":
+    def for_later(cls) -> "ExceptionInfo[_E]":
         """return an unfilled ExceptionInfo
         """
         return cls(None)
 
     @property
-    def type(self) -> "Type[BaseException]":
+    def type(self) -> "Type[_E]":
         """the exception class"""
         assert (
             self._excinfo is not None
         ), ".type can only be used after the context manager exits"
         return self._excinfo[0]
 
     @property
-    def value(self) -> BaseException:
+    def value(self) -> _E:
         """the exception value"""
         assert (
             self._excinfo is not None

src/_pytest/python_api.py

@@ -10,10 +10,13 @@
 from types import TracebackType
 from typing import Any
 from typing import Callable
+from typing import cast
+from typing import Generic
 from typing import Optional
 from typing import overload
 from typing import Pattern
 from typing import Tuple
+from typing import TypeVar
 from typing import Union
 
 from more_itertools.more import always_iterable
@@ -537,33 +540,35 @@ def _is_numpy_array(obj):
 
 # builtin pytest.raises helper
 
+_E = TypeVar("_E", bound=BaseException)
+
 
 @overload
 def raises(
-    expected_exception: Union["Type[BaseException]", Tuple["Type[BaseException]", ...]],
+    expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     *,
     match: Optional[Union[str, Pattern]] = ...  # noqa: W504 (SyntaxError in Python 3.5)
-) -> "RaisesContext":
+) -> "RaisesContext[_E]":
     ...
 
 
 @overload
 def raises(
-    expected_exception: Union["Type[BaseException]", Tuple["Type[BaseException]", ...]],
+    expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     func: Callable,
     *args: Any,
     match: Optional[str] = ...,
     **kwargs: Any
-) -> Optional[_pytest._code.ExceptionInfo]:
+) -> Optional[_pytest._code.ExceptionInfo[_E]]:
     ...
 
 
 def raises(
-    expected_exception: Union["Type[BaseException]", Tuple["Type[BaseException]", ...]],
+    expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     *args: Any,
     match: Optional[Union[str, Pattern]] = None,
     **kwargs: Any
-) -> Union["RaisesContext", Optional[_pytest._code.ExceptionInfo]]:
+) -> Union["RaisesContext[_E]", Optional[_pytest._code.ExceptionInfo[_E]]]:
     r"""
     Assert that a code block/function call raises ``expected_exception``
     or raise a failure exception otherwise.
@@ -702,28 +707,30 @@ def raises(
         try:
             func(*args[1:], **kwargs)
         except expected_exception:
-            return _pytest._code.ExceptionInfo.from_current()
+            # Cast to narrow the type to expected_exception (_E).
+            return cast(
+                _pytest._code.ExceptionInfo[_E],
+                _pytest._code.ExceptionInfo.from_current(),
+            )
     fail(message)
 
 
 raises.Exception = fail.Exception  # type: ignore
 
 
-class RaisesContext:
+class RaisesContext(Generic[_E]):
     def __init__(
         self,
-        expected_exception: Union[
-            "Type[BaseException]", Tuple["Type[BaseException]", ...]
-        ],
+        expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
         message: str,
         match_expr: Optional[Union[str, Pattern]] = None,
     ) -> None:
         self.expected_exception = expected_exception
         self.message = message
         self.match_expr = match_expr
-        self.excinfo = None  # type: Optional[_pytest._code.ExceptionInfo]
+        self.excinfo = None  # type: Optional[_pytest._code.ExceptionInfo[_E]]
 
-    def __enter__(self) -> _pytest._code.ExceptionInfo:
+    def __enter__(self) -> _pytest._code.ExceptionInfo[_E]:
         self.excinfo = _pytest._code.ExceptionInfo.for_later()
         return self.excinfo