blazio

A drop-in asyncio event loop for CPython, backed by zio.ev — the completion-based event loop from zio (io_uring on Linux, kqueue and IOCP elsewhere). Like uvloop, but on zio.ev instead of libuv, and with real asynchronous file I/O.

import asyncio
import blazio

async def main():
    reader, writer = await asyncio.open_connection("example.com", 80)
    writer.write(b"GET / HTTP/1.0\r\nHost: example.com\r\n\r\n")
    await writer.drain()
    print(await reader.read())
    writer.close()

asyncio.run(main(), loop_factory=blazio.new_event_loop)

That loop_factory argument is the only change; the rest of your program is standard asyncio.

Async files

blazio.open() is an aiofiles-compatible API over zio.ev's file completions — real asynchronous file I/O, not a thread pool.

async with blazio.open("access.log") as f:
    async for line in f:
        ...

Seekable files also support positional pread/pwrite.

Performance

About 1.15x uvloop on transport-based HTTP. Measured on one Linux machine (io_uring, CPython 3.12, ReleaseFast): 20 s wrk runs after a 30 s settle, 100 connections. benchmarks/ reproduces them.

Both drive the Transport/Protocol layer, which is what real servers (uvicorn, aiohttp, asyncio streams) run on. The low-level loop.sock_* API is a separate path that the three loops optimize very differently, so it is not benchmarked here.

benchmarks/mini_http_server.py — a minimal Protocol writing a fixed 1 KB response, with no framework in the path, so this is close to raw loop overhead:

loop req/s p50 p90
asyncio 101,428 0.91ms 1.03ms
uvloop 127,036 0.74ms 0.81ms
blazio 144,781 0.64ms 0.88ms

benchmarks/uvicorn_server.py — the same response through uvicorn. Most of each request is now framework Python that every loop pays equally, so all three drop sharply while the ratio between them holds:

loop req/s p50 p90
asyncio 23,501 4.02ms 4.27ms
uvloop 41,596 2.29ms 2.35ms
blazio 48,274 1.74ms 2.53ms

Install

pip install git+https://github.com/lalinsky/blazio

Requires CPython 3.12 or newer and Zig 0.16 on PATH.

From a checkout, with the tests:

pip install .
python -m pytest tests/

License

MIT. See LICENSE.

Project links

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Avatar for lukas.lalinsky from gravatar.com lukas.lalinsky

Credits

Author: Lukas Lalinsky

GitHub Statistics

Data verified by PyPI on Jul 23, 2026
The GitHub source repository was provided by the project maintainers and verified by PyPI at the time of upload. Stars, forks, and open issues/PRs are derived from that repository and have not been independently verified.

License expression

MIT
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Requires

Python >=3.12

Tags

asyncio event-loop io_uring zio uvloop

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