Windows: Psycopg cannot use the 'ProactorEventLoop' to run in async mode.

[sdancer75]

Under windows

psycopg.InterfaceError: Psycopg cannot use the 'ProactorEventLoop' to run in async mode. Please use a compatible event loop, for instance by running 'asyncio.run(..., loop_factory=asyncio.SelectorEventLoop(selectors.SelectSelector()))'

Windows defaults to ProactorEventLoop.

ProactorEventLoop is Windows' default, but doesn't work with psycopg (async PostgreSQL driver).

SelectorEventLoop works with psycopg on Windows

Solution

Under backend/api.py

if sys.platform == "win32":
    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

[linear]

SUNA-1071

[sdancer75]

The new version of Psycopg 3.3 does not support the ProactorEventLoop on Windows.

Even with the release of version 3.3 in late 2025, the official documentation and community issue trackers continue to specify that Psycopg remains incompatible with the ProactorEventLoop due to its reliance on specific file descriptor methods (add_reader() and add_writer()) that this loop does not provide.