The basics

AnyIO requires Python 3.8 or later to run. It is recommended that you set up a virtualenv when developing or playing around with AnyIO.


To install AnyIO, run:

pip install anyio

To install a supported version of Trio, you can install it as an extra like this:

pip install anyio[trio]

Running async programs

The simplest possible AnyIO program looks like this:

from anyio import run

async def main():
    print('Hello, world!')


This will run the program above on the default backend (asyncio). To run it on another supported backend, say Trio, you can use the backend argument, like so:

run(main, backend='trio')

But AnyIO code is not required to be run via run(). You can just as well use the native run() function of the backend library:

import sniffio
import trio
from anyio import sleep

async def main():
    await sleep(1)
    print("I'm running on", sniffio.current_async_library())

Changed in version 4.0.0: On the asyncio backend, now uses a back-ported version of asyncio.Runner on Pythons older than 3.11.

Backend specific options


  • options covered in the documentation of asyncio.Runner

  • use_uvloop (bool, default=False): Use the faster uvloop event loop implementation, if available (this is a shorthand for passing loop_factory=uvloop.new_event_loop, and is ignored if loop_factory is passed a value other than None)

Trio: options covered in the official documentation

Changed in version 3.2.0: The default value of use_uvloop was changed to False.

Changed in version 4.0.0: The policy option was replaced with loop_factory.

Using native async libraries

AnyIO lets you mix and match code written for AnyIO and code written for the asynchronous framework of your choice. There are a few rules to keep in mind however:

  • You can only use “native” libraries for the backend you’re running, so you cannot, for example, use a library written for Trio together with a library written for asyncio.

  • Tasks spawned by these “native” libraries on backends other than Trio are not subject to the cancellation rules enforced by AnyIO

  • Threads spawned outside of AnyIO cannot use to call asynchronous code