.. currentmodule:: asyncio
.. _asyncio-dev:
=======================
Developing with asyncio
=======================
Asynchronous programming is different from classic "sequential"
programming.
This page lists common mistakes and traps and explains how
to avoid them.
.. _asyncio-debug-mode:
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 :envvar:`PYTHONASYNCIODEBUG` environment variable to ``1``.
* Using the :ref:`Python Development Mode <devmode>`.
* Passing ``debug=True`` to :func:`asyncio.run`.
* Calling :meth:`loop.set_debug`.
In addition to enabling the debug mode, consider also:
* setting the log level of the :ref:`asyncio logger <asyncio-logger>` to
:py:data:`logging.DEBUG`, for example the following snippet of code
can be run at startup of the application::
logging.basicConfig(level=logging.DEBUG)
* configuring the :mod:`warnings` module to display
:exc:`ResourceWarning` warnings. One way of doing that is by
using the :option:`-W` ``default`` command line option.
When the debug mode is enabled:
* asyncio checks for :ref:`coroutines that were not awaited
<asyncio-coroutine-not-scheduled>` and logs them; this mitigates
the "forgotten await" pitfall.
* Many non-threadsafe asyncio APIs (such as :meth:`loop.call_soon` and
:meth:`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 100 milliseconds are logged. The
:attr:`loop.slow_callback_duration` attribute can be used to set the
minimum execution duration in seconds that is considered "slow".
.. _asyncio-multithreading:
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 :term:`callback` from another OS thread, the
:meth:`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 :meth:`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
:func:`run_coroutine_threadsafe` function should be used. It returns a
:class:`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 :meth:`loop.run_in_executor` method can be used with a
:class:`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
:mod:`multiprocessing`). The :ref:`asyncio-event-loop-methods`
section lists APIs that can read from pipes and watch file descriptors
without blocking the event loop. In addition, asyncio's
:ref:`Subprocess <asyncio-subprocess>` APIs provide a way to start a
process and communicate with it from the event loop. Lastly, the
aforementioned :meth:`loop.run_in_executor` method can also be used
with a :class:`concurrent.futures.ProcessPoolExecutor` to execute
code in a different process.
.. _asyncio-handle-blocking:
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 :meth:`loop.run_in_executor` method for more
details.
.. _asyncio-logger:
Logging
=======
asyncio uses the :mod:`logging` module and all logging is performed
via the ``"asyncio"`` logger.
The default log level is :py:data:`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 :ref:`blocking-handlers`.
.. _asyncio-coroutine-not-scheduled:
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 :meth:`asyncio.create_task`, asyncio
will emit a :exc:`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
:meth:`asyncio.create_task` function::
async def main():
await test()
Detect never-retrieved exceptions
=================================
If a :meth:`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
:ref:`Enable the debug mode <asyncio-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
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