remove monkeypatching TestClient interface

setup.py

@@ -37,7 +37,10 @@ def get_long_description():
     packages=find_packages(exclude=["tests*"]),
     package_data={"starlette": ["py.typed"]},
     include_package_data=True,
-    install_requires=["anyio>=3.0.0,<4"],
+    install_requires=[
+        "anyio>=3.0.0,<4",
+        "typing_extensions; python_version < '3.8'",
+    ],
     extras_require={
         "full": [
             "graphene; python_version<'3.10'",

starlette/testclient.py

@@ -6,6 +6,7 @@
 import json
 import math
 import queue
+import sys
 import types
 import typing
 from concurrent.futures import Future
@@ -18,6 +19,11 @@
 from starlette.types import Message, Receive, Scope, Send
 from starlette.websockets import WebSocketDisconnect
 
+if sys.version_info >= (3, 8):
+    from typing import TypedDict
+else:
+    from typing_extensions import TypedDict
+
 # Annotations for `Session.request()`
 Cookies = typing.Union[
     typing.MutableMapping[str, str], requests.cookies.RequestsCookieJar
@@ -91,11 +97,16 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         await instance(receive, send)
 
 
+class _AsyncBackend(TypedDict):
+    backend: str
+    backend_options: typing.Dict[str, typing.Any]
+
+
 class _ASGIAdapter(requests.adapters.HTTPAdapter):
     def __init__(
         self,
         app: ASGI3App,
-        async_backend: typing.Dict[str, typing.Any],
+        async_backend: _AsyncBackend,
         raise_server_exceptions: bool = True,
         root_path: str = "",
     ) -> None:
@@ -271,7 +282,10 @@ async def send(message: Message) -> None:
 
 class WebSocketTestSession:
     def __init__(
-        self, app: ASGI3App, scope: Scope, async_backend: typing.Dict[str, typing.Any]
+        self,
+        app: ASGI3App,
+        scope: Scope,
+        async_backend: _AsyncBackend,
     ) -> None:
         self.app = app
         self.scope = scope
@@ -381,11 +395,6 @@ def receive_json(self, mode: str = "text") -> typing.Any:
 
 class TestClient(requests.Session):
     __test__ = False  # For pytest to not discover this up.
-    #: These are the default options for the constructor arguments
-    async_backend: typing.Dict[str, typing.Any] = {
-        "backend": "asyncio",
-        "backend_options": {},
-    }
     task: "Future[None]"
 
     def __init__(
@@ -394,14 +403,13 @@ def __init__(
         base_url: str = "http://testserver",
         raise_server_exceptions: bool = True,
         root_path: str = "",
-        backend: typing.Optional[str] = None,
+        backend: str = "asyncio",
         backend_options: typing.Optional[typing.Dict[str, typing.Any]] = None,
     ) -> None:
         super().__init__()
-        self.async_backend = {
-            "backend": backend or self.async_backend["backend"],
-            "backend_options": backend_options or self.async_backend["backend_options"],
-        }
+        self.async_backend = _AsyncBackend(
+            backend=backend, backend_options=backend_options or {}
+        )
         if _is_asgi3(app):
             app = typing.cast(ASGI3App, app)
             asgi_app = app