urio

Async file I/O for Python that is actually asynchronous.

On Linux, urio submits reads and writes through io_uring and resolves completions on the event loop, with no thread parked per operation. On Windows it can do the same through overlapped I/O and an I/O completion port (opt-in, see below). Everywhere else — macOS, older kernels, sandboxes that block the syscalls — it falls back to a thread pool behind the same API, so one code path works on every platform.

The API follows aiofiles, which pioneered ergonomic async file access for asyncio by running the blocking syscalls in a thread pool — the only portable option at the time, and still the right fallback. urio keeps that familiar interface (which in turn mirrors the builtin open()), so switching is mostly changing the import.

What you get

• Real async reads and writes: io_uring on Linux, IOCP on Windows.
• The familiar file API: read, readline, readlines, write, writelines, seek, tell, truncate, flush, async iteration, text and binary modes.
• An async pathlib.Path: urio.Path with async stat, mkdir, unlink, rename, symlink_to, read_text/write_text/read_bytes/write_bytes, iterdir, glob, walk, and the full set of pure-path helpers.
• A thread-pool fallback wherever native async I/O is unavailable, same API.
• Runs GIL-free on free-threaded CPython (3.14t and later).

Requirements

• Python 3.12+, any OS.
• Linux 5.6+ for the io_uring backend.
• Windows 10/11 for the IOCP backend (opt-in via URIO_WINDOWS_IOCP=1; Windows otherwise uses the thread backend).

Anything else — macOS, older kernels, locked-down sandboxes — runs the thread backend automatically.

Installation

pip install urio

Prebuilt abi3 wheels (one per platform covers every CPython ≥ 3.12):

Platform	Arch	Backend
Linux (manylinux + musllinux)	x86_64, aarch64, armv7	io_uring
Windows	x64, x86	thread pool by default, IOCP opt-in
macOS	x86_64, arm64	thread pool

Building from source needs a Rust toolchain and maturin; the crate compiles on every OS:

pip install -e ".[dev]"
maturin develop

Usage

import asyncio
import urio


async def main():
    # Files — binary and text modes, just like the builtin open().
    async with urio.open('greeting.txt', 'w') as f:
        await f.write('hello\nworld\n')

    async with urio.open('greeting.txt') as f:
        async for line in f:
            print(line.rstrip())

    # Async pathlib.
    p = urio.Path('greeting.txt')
    print(await p.exists())
    print((await p.stat()).st_size)
    print(await p.read_text())


asyncio.run(main())

Async pathlib

urio.Path is an async pathlib.Path. Pure-path operations (/ joining, name, suffix, parent, with_suffix, …) are ordinary synchronous properties; everything that touches the filesystem is async and goes through the active backend:

base = urio.Path('/tmp/project')
await base.mkdir(parents=True, exist_ok=True)

cfg = base / 'config.toml'
await cfg.write_text('name = "urio"\n')
print(await cfg.read_text())
print(await cfg.exists(), await cfg.is_file(), (await cfg.stat()).st_size)

async for child in base.iterdir():
    print(child.name)

On Linux, stat/mkdir/unlink/rename/symlink_to and the read/write helpers are genuine io_uring submissions; directory listing and metadata tweaks use the thread pool.

Choosing a backend

urio picks the fastest available backend at runtime. You can force one:

urio.set_backend('auto')     # default
urio.set_backend('thread')   # force the thread pool
urio.set_backend('uring')    # require io_uring (raises if unavailable)
urio.set_backend('iocp')     # require IOCP (Windows, needs URIO_WINDOWS_IOCP=1)

or via the environment: URIO_BACKEND=thread python app.py. On Windows, setting URIO_WINDOWS_IOCP=1 is enough on its own — auto-detection then selects IOCP.

How it works

On Linux, a single ring is created per event loop. Its io_uring instance is registered with an eventfd, and that fd is handed to loop.add_reader, so the loop wakes whenever completions are ready. Each submission maps a user_data id to an asyncio.Future; on wake-up the driver drains the eventfd, reaps every completion, and resolves the matching futures. Everything runs on the loop thread — no executor, no extra threads.

On Windows with IOCP enabled, files are opened for overlapped I/O and associated with the completion port that asyncio's ProactorEventLoop already owns, so completions are again reaped on the loop thread. Same shape, native Windows machinery.

Whatever the kernel touches is kept alive until the completion is reaped: reads land in a buffer the kernel fills directly, and writes submit against the caller's immutable Python bytes object (held by reference), so neither direction copies. This also side-steps the hardest io_uring footgun: the kernel using a buffer that Python has freed or moved.

Benchmarks

benchmarks/bench_vs_aiofiles.py runs the full cartesian product of {buffered write, write+fsync, read} × {binary, text} × {2000×4 KB, 500×64 KB, 100×1 MB, 20×8 MB}, plus a streaming read of a 128 MB file. It tries hard to be fair: the cache is warmed for every contender, each contender writes to its own files (so nobody inherits another's writeback pressure), and it reports the median of N runs with the [min–max] range. The Linux charts below are medians on CPython 3.14.6, measured on a 4-core Intel i5-6500, 32 GB RAM, ext4 on a SATA HDD (the Windows numbers are from a separate 8-core box; full specs in the benchmarks doc). Treat them as indicative and run it on your hardware. Each bar is one operation × mode × size; colour is the file size, and a bar past the 1.0× line means urio beat aiofiles.

Linux, io_uring. Buffered writes win across sizes: submission batching for small files (one io_uring_enter per loop tick instead of one syscall per op), zero-copy submission for large ones. Reads win small/medium and hold up at large sizes too — reads are zero-copy (the kernel fills the returned bytes directly) and big reads fan out to io-wq workers so their copies run on multiple cores. Durable (fsync) writes converge on disk bandwidth and stay competitive:

Windows, IOCP. Real async I/O via overlapped I/O and a completion port, reaped on the event-loop thread, with zero-copy writes and reads. Buffered writes are at parity to winning, durable writes win, large text wins outright (1.3–1.9×). Binary reads still lose: a warm ReadFile completes synchronously, so the kernel copies inline on the loop thread. The full test suite and a concurrency/cancellation stress test pass on Windows CPython 3.14, both GIL and free-threaded builds:

Free-threaded Python (no-GIL). The native module declares gil_used = false, and urio has the same thread-pool mode aiofiles does — so on no-GIL builds you lose nothing: use the thread backend for warm reads (parity with aiofiles), and io_uring/IOCP for writes, fsync, and latency, where the batching wins persist. The chart isolates the native backends' warm-read edge as the now-unshackled thread pools catch up: on Linux the small-read edge narrows but stays a win at every size, while on Windows the IOCP large-text-read wins drop below parity without the GIL:

Summary vs aiofiles (medians, CPython 3.14):

• Linux writes win at small/medium sizes through submission batching (2.9–5.3×); large binary writes ride zero-copy submission (1.5× at 1 MB, parity at 8 MB), large text trails.
• Linux reads win small/medium (2.6–3.5×) and, with zero-copy reads plus the io-wq punt, hold parity at 1 MB and 0.7× at 8 MB; large text reads win (1.1–1.4×). Warm streaming is a clear win — binary 6.4 GB/s vs aiofiles's 3.3 (1.9×).
• Durable (fsync) writes converge on disk bandwidth and are competitive on both io_uring and IOCP. On ZFS every fsync forces a ZIL/txg commit, so large durable writes get ~24–71× slower — but aiofiles pays the same tax (speedup ~1.0×) and urio's batched fsync still wins on many small files. See the benchmarks doc.
• Windows IOCP: writes parity to winning, durable writes win (1.1–1.7×), large text wins (1.3–1.9×); binary reads lose (the warm ReadFile copy runs inline on the loop thread).
• The thread backend tracks aiofiles on both reads and writes, on every OS and in both GIL states — same strategy, used wherever io_uring or IOCP is unavailable or not selected.

Run it yourself: python benchmarks/bench_vs_aiofiles.py --markdown --json out.json (medians + raw samples). Methodology and full tables are in the benchmarks docs; raw sample data lives in benchmarks/data/.

Documentation

The full documentation lives at urio.readthedocs.io. ARCHITECTURE.md is the code-reader's tour: the ring/eventfd bridge, the zero-copy rules, linked chains, and the backend stack. To build the site locally:

pip install -e ".[docs]"
mkdocs serve

Scope & design notes

• Metadata ops without an io_uring opcode use the thread pool. io_uring covers the hot path — read, write, fsync/fdatasync, openat, close, statx, mkdirat, unlinkat, renameat, symlinkat, linkat — while a few rarer operations (directory listing, chmod, readlink, realpath) have no opcode and run on the thread pool even under the io_uring backend, behind the same async API.
• Text decoding is incremental. Reading a whole text file is one round-trip plus a single bulk decode, and sized read(n) fetches the whole request in one round-trip. Line-by-line iteration decodes incrementally (128 KB per refill), so it trails a C TextIOWrapper on pure line-streaming throughput; read binary and bulk-decode if that is your bottleneck. On free-threaded builds the codec runs in the executor so it parallelises across cores. Text streams are seekable in byte offsets, like io.TextIOWrapper.

License

MIT