cpython/Doc/library/asyncio-dev.rst

.. 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 :option:`-X` ``dev`` Python command line option.

* 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-treadsafe 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 100ms 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 callback from a different 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.


.. _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 block 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)


.. _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