Add Client Middleware Support

CHANGES/9732.feature.rst

@@ -0,0 +1,6 @@
+Added client middleware support -- by :user:`bdraco` and :user:`Dreamsorcerer`.
+
+This change allows users to add middleware to the client session and requests, enabling features like
+authentication, logging, and request/response modification without modifying the core
+request logic. Additionally, the ``session`` attribute was added to ``ClientRequest``,
+allowing middleware to access the session for making additional requests.

aiohttp/__init__.py

@@ -47,6 +47,7 @@
     WSServerHandshakeError,
     request,
 )
+from .client_middlewares import ClientHandlerType, ClientMiddlewareType
 from .compression_utils import set_zlib_backend
 from .connector import AddrInfoType, SocketFactoryType
 from .cookiejar import CookieJar, DummyCookieJar
@@ -157,6 +158,9 @@
     "NamedPipeConnector",
     "WSServerHandshakeError",
     "request",
+    # client_middleware
+    "ClientMiddlewareType",
+    "ClientHandlerType",
     # cookiejar
     "CookieJar",
     "DummyCookieJar",

aiohttp/client.py

@@ -72,6 +72,7 @@
     WSMessageTypeError,
     WSServerHandshakeError,
 )
+from .client_middlewares import ClientMiddlewareType, build_client_middlewares
 from .client_reqrep import (
     SSL_ALLOWED_TYPES,
     ClientRequest,
@@ -193,6 +194,7 @@ class _RequestOptions(TypedDict, total=False):
     auto_decompress: Union[bool, None]
     max_line_size: Union[int, None]
     max_field_size: Union[int, None]
+    middlewares: Optional[Tuple[ClientMiddlewareType, ...]]
 
 
 @frozen_dataclass_decorator
@@ -260,6 +262,7 @@ class ClientSession:
         "_default_proxy",
         "_default_proxy_auth",
         "_retry_connection",
+        "_middlewares",
     )
 
     def __init__(
@@ -292,6 +295,7 @@ def __init__(
         max_line_size: int = 8190,
         max_field_size: int = 8190,
         fallback_charset_resolver: _CharsetResolver = lambda r, b: "utf-8",
+        middlewares: Optional[Tuple[ClientMiddlewareType, ...]] = None,
     ) -> None:
         # We initialise _connector to None immediately, as it's referenced in __del__()
         # and could cause issues if an exception occurs during initialisation.
@@ -376,6 +380,7 @@ def __init__(
         self._default_proxy = proxy
         self._default_proxy_auth = proxy_auth
         self._retry_connection: bool = True
+        self._middlewares = middlewares
 
     def __init_subclass__(cls: Type["ClientSession"]) -> None:
         raise TypeError(
@@ -450,6 +455,7 @@ async def _request(
         auto_decompress: Optional[bool] = None,
         max_line_size: Optional[int] = None,
         max_field_size: Optional[int] = None,
+        middlewares: Optional[Tuple[ClientMiddlewareType, ...]] = None,
     ) -> ClientResponse:
         # NOTE: timeout clamps existing connect and read timeouts.  We cannot
         # set the default to None because we need to detect if the user wants
@@ -642,32 +648,33 @@ async def _request(
                         trust_env=self.trust_env,
                     )
 
-                    # connection timeout
-                    try:
-                        conn = await self._connector.connect(
-                            req, traces=traces, timeout=real_timeout
+                    # Core request handler - now includes connection logic
+                    async def _connect_and_send_request(
+                        req: ClientRequest,
+                    ) -> ClientResponse:
+                        # connection timeout
+                        assert self._connector is not None
+                        try:
+                            conn = await self._connector.connect(
+                                req, traces=traces, timeout=real_timeout
+                            )
+                        except asyncio.TimeoutError as exc:
+                            raise ConnectionTimeoutError(
+                                f"Connection timeout to host {req.url}"
+                            ) from exc
+
+                        assert conn.protocol is not None
+                        conn.protocol.set_response_params(
+                            timer=timer,
+                            skip_payload=req.method in EMPTY_BODY_METHODS,
+                            read_until_eof=read_until_eof,
+                            auto_decompress=auto_decompress,
+                            read_timeout=real_timeout.sock_read,
+                            read_bufsize=read_bufsize,
+                            timeout_ceil_threshold=self._connector._timeout_ceil_threshold,
+                            max_line_size=max_line_size,
+                            max_field_size=max_field_size,
                         )
-                    except asyncio.TimeoutError as exc:
-                        raise ConnectionTimeoutError(
-                            f"Connection timeout to host {url}"
-                        ) from exc
-
-                    assert conn.transport is not None
-
-                    assert conn.protocol is not None
-                    conn.protocol.set_response_params(
-                        timer=timer,
-                        skip_payload=method in EMPTY_BODY_METHODS,
-                        read_until_eof=read_until_eof,
-                        auto_decompress=auto_decompress,
-                        read_timeout=real_timeout.sock_read,
-                        read_bufsize=read_bufsize,
-                        timeout_ceil_threshold=self._connector._timeout_ceil_threshold,
-                        max_line_size=max_line_size,
-                        max_field_size=max_field_size,
-                    )
-
-                    try:
                         try:
                             resp = await req.send(conn)
                             try:
@@ -678,6 +685,30 @@ async def _request(
                         except BaseException:
                             conn.close()
                             raise
+                        return resp
+
+                    # Apply middleware (if any) - per-request middleware overrides session middleware
+                    effective_middlewares = (
+                        self._middlewares if middlewares is None else middlewares
+                    )
+
+                    if effective_middlewares:
+                        handler = build_client_middlewares(
+                            _connect_and_send_request, effective_middlewares
+                        )
+                    else:
+                        handler = _connect_and_send_request
+
+                    try:
+                        resp = await handler(req)
+                    # Client connector errors should not be retried
+                    except (
+                        ConnectionTimeoutError,
+                        ClientConnectorError,
+                        ClientConnectorCertificateError,
+                        ClientConnectorSSLError,
+                    ):
+                        raise
                     except (ClientOSError, ServerDisconnectedError):
                         if retry_persistent_connection:
                             retry_persistent_connection = False

aiohttp/client_middlewares.py

@@ -0,0 +1,58 @@
+"""Client middleware support."""
+
+from collections.abc import Awaitable, Callable
+
+from .client_reqrep import ClientRequest, ClientResponse
+
+__all__ = ("ClientMiddlewareType", "ClientHandlerType", "build_client_middlewares")
+
+# Type alias for client request handlers - functions that process requests and return responses
+ClientHandlerType = Callable[[ClientRequest], Awaitable[ClientResponse]]
+
+# Type for client middleware - similar to server but uses ClientRequest/ClientResponse
+ClientMiddlewareType = Callable[
+    [ClientRequest, ClientHandlerType], Awaitable[ClientResponse]
+]
+
+
+def build_client_middlewares(
+    handler: ClientHandlerType,
+    middlewares: tuple[ClientMiddlewareType, ...],
+) -> ClientHandlerType:
+    """
+    Apply middlewares to request handler.
+
+    The middlewares are applied in reverse order, so the first middleware
+    in the list wraps all subsequent middlewares and the handler.
+
+    This implementation avoids using partial/update_wrapper to minimize overhead
+    and doesn't cache to avoid holding references to stateful middleware.
+    """
+    if not middlewares:
+        return handler
+
+    # Optimize for single middleware case
+    if len(middlewares) == 1:
+        middleware = middlewares[0]
+
+        async def single_middleware_handler(req: ClientRequest) -> ClientResponse:
+            return await middleware(req, handler)
+
+        return single_middleware_handler
+
+    # Build the chain for multiple middlewares
+    current_handler = handler
+
+    for middleware in reversed(middlewares):
+        # Create a new closure that captures the current state
+        def make_wrapper(
+            mw: ClientMiddlewareType, next_h: ClientHandlerType
+        ) -> ClientHandlerType:
+            async def wrapped(req: ClientRequest) -> ClientResponse:
+                return await mw(req, next_h)
+
+            return wrapped
+
+        current_handler = make_wrapper(middleware, current_handler)
+
+    return current_handler

aiohttp/client_reqrep.py

@@ -210,6 +210,11 @@ class ClientRequest:
     auth = None
     response = None
 
+    # These class defaults help create_autospec() work correctly.
+    # If autospec is improved in future, maybe these can be removed.
+    url = URL()
+    method = "GET"
+
     __writer: Optional["asyncio.Task[None]"] = None  # async task for streaming data
     _continue = None  # waiter future for '100 Continue' response
 
@@ -362,6 +367,16 @@ def request_info(self) -> RequestInfo:
             RequestInfo, (self.url, self.method, headers, self.original_url)
         )
 
+    @property
+    def session(self) -> "ClientSession":
+        """Return the ClientSession instance.
+
+        This property provides access to the ClientSession that initiated
+        this request, allowing middleware to make additional requests
+        using the same session.
+        """
+        return self._session
+
     def update_host(self, url: URL) -> None:
         """Update destination host, port and connection type (ssl)."""
         # get host/port