aio-libs/aiohttp · error · RuntimeError

Named Pipes only available in proactor loop under windows

Error message

Named Pipes only available in proactor loop under windows

What it means

Raised by NamedPipeConnector.__init__ when the running event loop is not a ProactorEventLoop. Windows named pipes are only implemented on the proactor loop, so on SelectorEventLoop (the default on non-Windows, or selected explicitly on Windows) the constructor refuses instead of failing later. This is a RuntimeError raised at construction time, before any request is attempted.

Source

Thrown at aiohttp/connector.py:1756

    def __init__(
        self,
        path: str,
        force_close: bool = False,
        keepalive_timeout: _SENTINEL | float | None = sentinel,
        limit: int = 100,
        limit_per_host: int = 0,
    ) -> None:
        super().__init__(
            force_close=force_close,
            keepalive_timeout=keepalive_timeout,
            limit=limit,
            limit_per_host=limit_per_host,
        )
        if not isinstance(
            self._loop,
            asyncio.ProactorEventLoop,  # type: ignore[attr-defined]
        ):
            raise RuntimeError(
                "Named Pipes only available in proactor loop under windows"
            )
        self._path = path

    @property
    def path(self) -> str:
        """Path to the named pipe."""
        return self._path

    async def _create_connection(
        self, req: ClientRequest, traces: list["Trace"], timeout: "ClientTimeout"
    ) -> ResponseHandler:
        try:
            async with ceil_timeout(
                timeout.sock_connect, ceil_threshold=timeout.ceil_threshold
            ):
                _, proto = await self._loop.create_pipe_connection(  # type: ignore[attr-defined]
                    self._factory, self._path

View on GitHub (pinned to c0ef574e29)

Solutions

1. On Windows, set the proactor loop before constructing: `asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())`.
2. Only construct NamedPipeConnector on Windows after checking `sys.platform == 'win32'` and the loop type.
3. On non-Windows, use UnixConnector (Unix socket) or the default TCPConnector instead.

Example fix

# before (Linux)
connector = aiohttp.NamedPipeConnector(r'\\\\.\\pipe\\foo')
# after
import sys, asyncio
if sys.platform == 'win32':
    asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
    connector = aiohttp.NamedPipeConnector(r'\\\\.\\pipe\\foo')
else:
    raise RuntimeError('Named pipes are Windows-only')

import sys, asyncio

def can_use_named_pipe() -> bool:
    if sys.platform != 'win32':
        return False
    return isinstance(asyncio.get_event_loop(), asyncio.ProactorEventLoop)

When it happens

Trigger: Instantiating NamedPipeConnector on Linux/macOS, or on Windows while a SelectorEventLoop is active, or inside an environment that forces the selector loop.

Common situations: Cross-platform code that unconditionally builds a NamedPipeConnector. asyncio.get_event_loop() returning a selector loop in older Python on Windows. Tests running on a non-Windows CI runner.

Related errors

• Named Pipes only available in proactor loop under windows
• Cannot initialize a TLS-in-TLS connection to host {req.url.h
• Session and connector have to use same event loop
• keepalive_timeout cannot be set if force_close is True
• Connector is closed.

AI-assisted analysis of aio-libs/aiohttp@c0ef574e29 (2026-08-04). Data as JSON: /data/errors/11b7d509991059ba.json. Report an issue: GitHub.