uvloop — Ultra-Fast asyncio Event Loop for Python

Introduction

uvloop is a fast, drop-in replacement for the built-in asyncio event loop in Python. It is implemented in Cython and wraps libuv, the same high-performance I/O library that powers Node.js. Switching to uvloop typically requires only two lines of code and can double or triple the throughput of asyncio-based applications.

What uvloop Does

• Replaces the default asyncio event loop with a libuv-backed implementation
• Accelerates TCP, UDP, and Unix socket operations for async Python code
• Provides full compatibility with the standard asyncio API
• Improves performance of frameworks like aiohttp, Sanic, and FastAPI
• Supports DNS resolution, subprocess management, and signal handling

Architecture Overview

uvloop is written in Cython and compiles to a C extension module. It wraps libuv handles and requests to implement the asyncio event loop interface. Socket I/O, timers, signal watchers, and child process management all delegate to libuv's non-blocking, cross-platform primitives. Because it follows the asyncio abstract base class contract, existing coroutines and libraries work without modification.

Self-Hosting & Configuration

• Install with pip install uvloop (requires Python 3.8+)
• Activate globally with asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
• Or use uvloop.install() as a shortcut in Python 3.12+
• No configuration files are needed; it is a runtime swap
• Works on Linux and macOS (Windows is not supported by libuv's full feature set in this context)

Key Features

• Two to four times faster than the default asyncio loop on network benchmarks
• Zero API changes required for existing asyncio code
• Built on libuv, a proven C library with years of production use in Node.js
• Handles millions of connections efficiently with low memory overhead
• Used by Uvicorn, the default ASGI server for FastAPI and Starlette

Comparison with Similar Tools

• asyncio (default loop) — Standard library implementation; uvloop is a faster drop-in alternative
• Trio — Alternative async library with structured concurrency; uvloop stays within the asyncio ecosystem
• curio — Minimal async kernel; uvloop targets the standard asyncio interface for broader compatibility
• libuv (raw) — C library for async I/O; uvloop wraps it and exposes it through Python's asyncio API

FAQ

Q: Does uvloop work on Windows? A: No. libuv on Windows uses IOCP which has different semantics. uvloop officially supports Linux and macOS.

Q: Will my existing asyncio code break? A: No. uvloop implements the full asyncio event loop interface. Existing coroutines, tasks, and libraries work without changes.

Q: How do I use uvloop with Uvicorn? A: Uvicorn uses uvloop by default when it is installed. Just pip install uvloop and Uvicorn will auto-detect it.

Q: Is there a performance cost to using Cython? A: The Cython compilation step happens at package build time. At runtime, uvloop executes as native C code with no interpretation overhead.

Sources

• https://github.com/MagicStack/uvloop
• https://uvloop.readthedocs.io