Improve typing of exception handlers

setup.cfg

@@ -3,6 +3,7 @@ ignore = W503, E203, B305
 max-line-length = 88
 
 [mypy]
+python_version = 3.6
 disallow_untyped_defs = True
 ignore_missing_imports = True
 no_implicit_optional = True

starlette/applications.py

@@ -10,6 +10,18 @@
 from starlette.routing import BaseRoute, Router
 from starlette.types import ASGIApp, Receive, Scope, Send
 
+_ExcKey = typing.TypeVar(
+    "_ExcKey",
+    bound=typing.Union[int, typing.Type[Exception]],
+    contravariant=True,
+)
+# Breaking out Exception to a bounded TypeVar here allows defining a narrower type in
+# handlers, like HTTPException.
+_HandledException = typing.TypeVar("_HandledException", bound=Exception)
+_ExcHandler = typing.Callable[
+    [Request, _HandledException], typing.Union[Response, typing.Awaitable[Response]]
+]
+
 
 class Starlette:
     """
@@ -44,13 +56,7 @@ def __init__(
         routes: typing.Optional[typing.Sequence[BaseRoute]] = None,
         middleware: typing.Optional[typing.Sequence[Middleware]] = None,
         exception_handlers: typing.Optional[
-            typing.Mapping[
-                typing.Any,
-                typing.Callable[
-                    [Request, Exception],
-                    typing.Union[Response, typing.Awaitable[Response]],
-                ],
-            ]
+          typing.Mapping[_ExcKey, _ExcHandler]
         ] = None,
         on_startup: typing.Optional[typing.Sequence[typing.Callable]] = None,
         on_shutdown: typing.Optional[typing.Sequence[typing.Callable]] = None,
@@ -78,9 +84,7 @@ def __init__(
     def build_middleware_stack(self) -> ASGIApp:
         debug = self.debug
         error_handler = None
-        exception_handlers: typing.Dict[
-            typing.Any, typing.Callable[[Request, Exception], Response]
-        ] = {}
+        exception_handlers = {}
 
         for key, value in self.exception_handlers.items():
             if key in (500, Exception):
@@ -170,8 +174,8 @@ def add_middleware(
 
     def add_exception_handler(
         self,
-        exc_class_or_status_code: typing.Union[int, typing.Type[Exception]],
-        handler: typing.Callable,
+        exc_class_or_status_code: _ExcKey,
+        handler: _ExcHandler,
     ) -> None:  # pragma: no cover
         self.exception_handlers[exc_class_or_status_code] = handler
         self.middleware_stack = self.build_middleware_stack()

tests/test_applications.py

@@ -89,7 +89,7 @@ async def websocket_endpoint(session):
     Middleware(TrustedHostMiddleware, allowed_hosts=["testserver", "*.example.org"])
 ]
 
-app = Starlette(
+app = Starlette(  # type: ignore[type-var]
     routes=[
         Route("/func", endpoint=func_homepage),
         Route("/async", endpoint=async_homepage),