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:
Setting the
PYTHONASYNCIODEBUG
environment variable to1
.Using the Python Development Mode.
Passing
debug=True
toasyncio.run()
.Calling
loop.set_debug()
.
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:logging.basicConfig(level=logging.DEBUG)
configuring the
warnings
module to displayResourceWarning
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-threadsafe asyncio APIs (such as
loop.call_soon()
andloop.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 another 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.:
loop.call_soon_threadsafe(fut.cancel)
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.
There is currently no way to schedule coroutines or callbacks directly
from a different process (such as one started with
multiprocessing
). The Event Loop Methods
section lists APIs that can read from pipes and watch file descriptors
without blocking the event loop. In addition, asyncio’s
Subprocess APIs provide a way to start a
process and communicate with it from the event loop. Lastly, the
aforementioned loop.run_in_executor()
method can also be used
with a concurrent.futures.ProcessPoolExecutor
to execute
code in a different process.
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 the OS thread with the
event loop. See the loop.run_in_executor()
method for more
details.
Logging¶
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:
logging.getLogger("asyncio").setLevel(logging.WARNING)
Network logging can block the event loop. It is recommended to use a separate thread for handling logs or use non-blocking IO. For example, see Dealing with handlers that block.
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():
test()
asyncio.run(main())
Output:
test.py:7: RuntimeWarning: coroutine 'test' was never awaited
test()
Output in debug mode:
test.py:7: RuntimeWarning: coroutine 'test' was never awaited
Coroutine created at (most recent call last)
File "../t.py", line 9, in <module>
asyncio.run(main(), debug=True)
< .. >
File "../t.py", line 7, in main
test()
test()
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():
asyncio.create_task(bug())
asyncio.run(main())
Output:
Task exception was never retrieved
future: <Task finished coro=<bug() done, defined at test.py:3>
exception=Exception('not consumed')>
Traceback (most recent call last):
File "test.py", line 4, in bug
raise Exception("not consumed")
Exception: not consumed
Enable the debug mode to get the traceback where the task was created:
asyncio.run(main(), debug=True)
Output in debug mode:
Task exception was never retrieved
future: <Task finished coro=<bug() done, defined at test.py:3>
exception=Exception('not consumed') created at asyncio/tasks.py:321>
source_traceback: Object created at (most recent call last):
File "../t.py", line 9, in <module>
asyncio.run(main(), debug=True)
< .. >
Traceback (most recent call last):
File "../t.py", line 4, in bug
raise Exception("not consumed")
Exception: not consumed