WindowsSelectorEventLoopPolicy set before uvicorn.run() is ignored in 0.36.0+ (worked in 0.34.2)

[ukalwa]

Setting asyncio.WindowsSelectorEventLoopPolicy() before calling uvicorn.run() no longer works in uvicorn 0.36.0+, breaking compatibility with libraries that require SelectorEventLoop on Windows (e.g., psycopg3).

Expected Behavior:
When the event loop policy is set before uvicorn.run(), uvicorn should respect it and create the appropriate event loop type.

Actual Behavior:
✅ uvicorn 0.34.2: Correctly uses _WindowsSelectorEventLoop
❌ uvicorn 0.37.0: Ignores the policy and uses ProactorEventLoop

MRE:

"""
Test with uvicorn 0.34.2 (works):
    uv run --script --with "uvicorn[standard]==0.34.2" --with fastapi \\
        --with "psycopg[binary]" uvicorn_issue_mre.py

Test with uvicorn 0.37.0 (fails):
    uv run --script --with "uvicorn[standard]==0.37.0" --with fastapi \\
        --with "psycopg[binary]" uvicorn_issue_mre.py

Note: psycopg will fail on event loop check before attempting DB connection.
"""

import asyncio
import sys
from collections.abc import AsyncIterator
from contextlib import asynccontextmanager

import psycopg
from fastapi import FastAPI

# Global connection - will be None but psycopg checks event loop on import/connect
db_conn: psycopg.AsyncConnection[tuple] | None = None


@asynccontextmanager
async def lifespan(app: FastAPI) -> AsyncIterator[None]:
    """FastAPI lifespan to manage database connection"""
    global db_conn
    loop = asyncio.get_running_loop()
    loop_type = type(loop).__name__
    print(f"Event loop type: {loop_type}")  # noqa: T201

    # Try to create async connection - psycopg checks event loop type here
    # This will fail on ProactorEventLoop before even trying to connect
    try:
        db_conn = await psycopg.AsyncConnection.connect(
            "postgresql://user:pass@localhost/db",
            autocommit=True,
            connect_timeout=1,
        )
        print("✓ Database connected successfully")  # noqa: T201
    except psycopg.InterfaceError as e:
        print(f"✗ psycopg InterfaceError (expected): {e}")  # noqa: T201
        raise
    except Exception as e:
        # Connection will fail but that's OK - we're testing event loop
        print(f"Connection failed (OK for test): {type(e).__name__}")  # noqa: T201

    yield

    if db_conn:
        await db_conn.close()
        print("✓ Database connection closed")  # noqa: T201


def create_app() -> FastAPI:
    """Factory function to create the FastAPI app"""
    app = FastAPI(lifespan=lifespan)

    @app.get("/")
    async def root() -> dict[str, str]:
        return {"message": "Hello World"}

    return app


if __name__ == "__main__":
    import uvicorn

    print(f"Python version: {sys.version}")  # noqa: T201
    print(f"uvicorn version: {uvicorn.__version__}")  # noqa: T201
    print(f"Platform: {sys.platform}")  # noqa: T201

    # On Windows, set the event loop policy BEFORE calling uvicorn.run()
    # This works in uvicorn 0.34.2 but fails in 0.36.0+
    if sys.platform.startswith("win"):
        print("Setting WindowsSelectorEventLoopPolicy...")  # noqa: T201
        asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

    print("Starting uvicorn...")  # noqa: T201
    uvicorn.run(
        "uvicorn_issue_mre:create_app",
        host="127.0.0.1",
        port=8000,
        factory=True,
    )

[nggit]

Note: asyncio.set_event_loop_policy will be removed in Python 3.16 https://docs.python.org/3/library/asyncio-policy.html .

To use SelectorEventLoop on Windows, my intuition tells me to try --loop asyncio:SelectorEventLoop even though I haven't tested it.

[ukalwa]

That did not work unfortunately.

File "/path/to/project/asgi.py", line 84, in <module>
    uvicorn.run(
File "/path/to/project/.venv/Lib/site-packages/uvicorn/main.py", line 580, in run
    server.run()
File "/path/to/project/.venv/Lib/site-packages/uvicorn/server.py", line 65, in run
    self.config.setup_event_loop()
File "/path/to/project/.venv/Lib/site-packages/uvicorn/config.py", line 476, in setup_event_loop
    loop_setup: Callable | None = import_from_string(LOOP_SETUPS[self.loop])
                                                     ~~~~~~~~~~~^^^^^^^^^^^
KeyError: 'asyncio:SelectorEventLoop'

Loop setups seem to only support these 4 options

LOOP_SETUPS: dict[LoopSetupType, str | None] = {
    "none": None,
    "auto": "uvicorn.loops.auto:auto_loop_setup",
    "asyncio": "uvicorn.loops.asyncio:asyncio_setup",
    "uvloop": "uvicorn.loops.uvloop:uvloop_setup",
}

[Kludex]

You are not on the latest version.

[ukalwa]

You are right, my bad. It works on the latest version, thanks!

[baltika10]

I suggest also to document it a bit more explicitly in https://uvicorn.dev/concepts/event-loop/#custom-event-loop

[Kludex]

PR welcome.

[baltika10]

PR is created #2925