Developing with asyncio

Asynchronous programming is different from classic “sequential” programming.

This page lists common mistakes and traps and explains how to avoid them.

Debug Mode

By default asyncio runs in production mode. In order to ease the development asyncio has a debug mode.

There are several ways to enable asyncio debug mode:

In addition to enabling the debug mode, consider also:

  • setting the log level of the asyncio logger to logging.DEBUG, for example the following snippet of code can be run at startup of the application:

  • configuring the warnings module to display ResourceWarning warnings. One way of doing that is by using the -W default command line option.

When the debug mode is enabled:

  • asyncio checks for coroutines that were not awaited and logs them; this mitigates the “forgotten await” pitfall.
  • Many non-treadsafe asyncio APIs (such as loop.call_soon() and loop.call_at() methods) raise an exception if they are called from a wrong thread.
  • The execution time of the I/O selector is logged if it takes too long to perform an I/O operation.
  • Callbacks taking longer than 100ms are logged. The loop.slow_callback_duration attribute can be used to set the minimum execution duration in seconds that is considered “slow”.

Concurrency and Multithreading

An event loop runs in a thread (typically the main thread) and executes all callbacks and Tasks in its thread. While a Task is running in the event loop, no other Tasks can run in the same thread. When a Task executes an await expression, the running Task gets suspended, and the event loop executes the next Task.

To schedule a callback from a different OS thread, the loop.call_soon_threadsafe() method should be used. Example:

loop.call_soon_threadsafe(callback, *args)

Almost all asyncio objects are not thread safe, which is typically not a problem unless there is code that works with them from outside of a Task or a callback. If there’s a need for such code to call a low-level asyncio API, the loop.call_soon_threadsafe() method should be used, e.g.:


To schedule a coroutine object from a different OS thread, the run_coroutine_threadsafe() function should be used. It returns a concurrent.futures.Future to access the result:

async def coro_func():
     return await asyncio.sleep(1, 42)

# Later in another OS thread:

future = asyncio.run_coroutine_threadsafe(coro_func(), loop)
# Wait for the result:
result = future.result()

To handle signals and to execute subprocesses, the event loop must be run in the main thread.

The loop.run_in_executor() method can be used with a concurrent.futures.ThreadPoolExecutor to execute blocking code in a different OS thread without blocking the OS thread that the event loop runs in.

Running Blocking Code

Blocking (CPU-bound) code should not be called directly. For example, if a function performs a CPU-intensive calculation for 1 second, all concurrent asyncio Tasks and IO operations would be delayed by 1 second.

An executor can be used to run a task in a different thread or even in a different process to avoid blocking block the OS thread with the event loop. See the loop.run_in_executor() method for more details.


asyncio uses the logging module and all logging is performed via the "asyncio" logger.

The default log level is logging.INFO, which can be easily adjusted:


Detect never-awaited coroutines

When a coroutine function is called, but not awaited (e.g. coro() instead of await coro()) or the coroutine is not scheduled with asyncio.create_task(), asyncio will emit a RuntimeWarning:

import asyncio

async def test():
    print("never scheduled")

async def main():

Output: RuntimeWarning: coroutine 'test' was never awaited

Output in debug mode: RuntimeWarning: coroutine 'test' was never awaited
Coroutine created at (most recent call last)
  File "../", line 9, in <module>, debug=True)

  < .. >

  File "../", line 7, in main

The usual fix is to either await the coroutine or call the asyncio.create_task() function:

async def main():
    await test()

Detect never-retrieved exceptions

If a Future.set_exception() is called but the Future object is never awaited on, the exception would never be propagated to the user code. In this case, asyncio would emit a log message when the Future object is garbage collected.

Example of an unhandled exception:

import asyncio

async def bug():
    raise Exception("not consumed")

async def main():


Task exception was never retrieved
future: <Task finished coro=<bug() done, defined at>
  exception=Exception('not consumed')>

Traceback (most recent call last):
  File "", line 4, in bug
    raise Exception("not consumed")
Exception: not consumed

Enable the debug mode to get the traceback where the task was created:, debug=True)

Output in debug mode:

Task exception was never retrieved
future: <Task finished coro=<bug() done, defined at>
    exception=Exception('not consumed') created at asyncio/>

source_traceback: Object created at (most recent call last):
  File "../", line 9, in <module>, debug=True)

< .. >

Traceback (most recent call last):
  File "../", line 4, in bug
    raise Exception("not consumed")
Exception: not consumed