An event loop for asyncio written in Rust

rsloop is a PyO3-based asyncio event loop implemented in Rust.

Each rsloop.Loop owns a dedicated Rust runtime thread for loop coordination and I/O work. That thread runs an rsloop-specialized vibeio runtime, using io_uring on Linux, IOCP on Windows, and native kqueue readiness on macOS. Native-stream TCP reads and Unix-domain socket reads run on that runtime. On Unix, generic TCP protocol readers use a second vibeio runtime on the Python loop thread (io_uring on Linux), avoiding cross-thread delivery of each read. Non-TLS accepts run on either runtime depending on where the server starts. Python callbacks, tasks, and coroutines run on the thread that calls run_forever() or run_until_complete() (usually the main Python thread).

The package exposes:

• a native extension module at rsloop._loop
• a Python wrapper in python/rsloop/__init__.py
• rsloop.Loop, rsloop.EventLoopPolicy, rsloop.new_event_loop(), rsloop.run(...), rsloop.install(), rsloop.uninstall(), and rsloop.build_info()

Repository metadata supports CPython 3.10 through 3.15. The native runtime requires Linux 6.1+, macOS 13+, or Windows 10+ so its hot paths can rely on modern completion, timer, and scheduler primitives. Free-threaded CPython (3.14t) is supported: the extension declares gil_used = false, so importing it no longer re-enables the GIL. See Free-Threaded CPython for what that does and does not buy you.

Documentation

Project documentation now lives in docs/.

If you are new to the repository, start with:

• docs/index.md
• docs/getting-started.md
• docs/supported-features.md
• docs/fast-streams.md
• docs/free-threading.md
• docs/rust-extensions.md
• docs/how-it-works.md
• docs/project-structure.md
• docs/development.md for building, testing, and profiling

To browse the docs locally with MkDocs:

uvx --from mkdocs mkdocs serve

Install

From PyPI:

pip install rsloop

With uv:

uv add rsloop

From conda-forge, using pixi:

pixi add rsloop

Usage

Simple entry point:

import rsloop


async def main(): ...


rsloop.run(main())

Install as the default asyncio event loop policy:

import asyncio
import rsloop

rsloop.install()
try:
    asyncio.run(main())
finally:
    rsloop.uninstall()

Manual loop creation also works:

import asyncio
import rsloop

loop = rsloop.new_event_loop()
asyncio.set_event_loop(loop)
try:
    loop.run_until_complete(...)
finally:
    asyncio.set_event_loop(None)
    loop.close()

Importing rsloop also patches asyncio.set_event_loop() so Python 3.10 can accept an rsloop.Loop instance, matching the behavior exercised by tests/test_run.py.

Examples

Run the repository examples from the project root:

uv run python examples/01_basics.py
uv run python examples/02_fd_and_sockets.py
uv run python examples/03_streams.py
uv run python examples/04_unix_and_accepted_socket.py
uv run python examples/05_pipes_signals_subprocesses.py

Example files: examples/01_basics.py, examples/02_fd_and_sockets.py, examples/03_streams.py, examples/04_unix_and_accepted_socket.py, examples/05_pipes_signals_subprocesses.py.

The repository also includes:

• examples/fastapi_service.py for running the same FastAPI app on stdlib asyncio, uvloop, or rsloop
• examples/granian_service.py for registering rsloop as Granian's worker event loop
• benches/compare_event_loops.py for callback, task, and TCP stream comparisons
• benches/compare_granian.py for Granian HTTP throughput and latency comparisons with oha

Benchmarks

See benchmark results and reproduction commands.

Acknowledgements

rsloop builds on the Python asyncio model and is implemented with PyO3 on the Rust side. Runtime and socket I/O are powered by vibeio.

License

This project is licensed under the Apache License, Version 2.0. See LICENSE for the full text.