Working with threads

Practical asynchronous applications occasionally need to run network, file or computationally expensive operations. Such operations would normally block the asynchronous event loop, leading to performance issues. To solution is to run such code in worker threads. Using worker threads lets the event loop continue running other tasks while the worker thread runs the blocking call.


Do not spawn too many threads, as the context switching overhead may cause your system to slow down to a crawl. A few dozen threads should be fine, but hundreds are probably bad. Consider using AnyIO’s semaphores to limit the maximum number of threads.

Running a function in a worker thread

To run a (synchronous) callable in a worker thread:

import time

from anyio import run_in_thread, run

async def main():
    await run_in_thread(time.sleep, 5)


Calling asynchronous code from a worker thread

If you need to call a coroutine function from a worker thread, you can do this:

from anyio import run_async_from_thread, sleep, run_in_thread, run

def blocking_function():
    run_async_from_thread(sleep, 5)

async def main():
    await run_in_thread(blocking_function)



The worker thread must have been spawned using run_in_thread() for this to work.