cpython/Doc/library/asyncio-dev.rst

.. currentmodule:: asyncio

.. _asyncio-dev:

Develop with asyncio
====================

Asynchronous programming is different than classical "sequential" programming.
This page lists common traps and explains how to avoid them.


.. _asyncio-debug-mode:

Debug mode of asyncio
---------------------

The implementation of :mod:`asyncio` has been written for performance.
In order to ease the development of asynchronous code, you may wish to
enable *debug mode*.

To enable all debug checks for an application:

* Enable the asyncio debug mode globally by setting the environment variable
  :envvar:`PYTHONASYNCIODEBUG` to ``1``, using ``-X dev`` command line option
  (see the :option:`-X` option), or by calling
  :meth:`loop.set_debug`.
* Set the log level of the :ref:`asyncio logger <asyncio-logger>` to
  :py:data:`logging.DEBUG`. For example, call
  ``logging.basicConfig(level=logging.DEBUG)`` at startup.
* Configure the :mod:`warnings` module to display :exc:`ResourceWarning`
  warnings. For example, use the ``-Wdefault`` command line option of Python to
  display them.

Examples debug checks:

* Log :ref:`coroutines defined but never "yielded from"
  <asyncio-coroutine-not-scheduled>`
* :meth:`loop.call_soon` and :meth:`loop.call_at` methods
  raise an exception if they are called from the wrong thread.
* Log the execution time of the selector
* Log callbacks taking more than 100 ms to be executed. The
  :attr:`loop.slow_callback_duration` attribute is the minimum
  duration in seconds of "slow" callbacks.
* :exc:`ResourceWarning` warnings are emitted when transports and event loops
  are :ref:`not closed explicitly <asyncio-close-transports>`.

.. versionchanged:: 3.7

   The new ``-X dev`` command line option can now also be used to enable
   the debug mode.

.. seealso::

   The :meth:`loop.set_debug` method and the :ref:`asyncio logger
   <asyncio-logger>`.


Cancellation
------------

Cancellation of tasks is not common in classic programming. In asynchronous
programming, not only is it something common, but you have to prepare your
code to handle it.

Futures and tasks can be cancelled explicitly with their :meth:`Future.cancel`
method. The :func:`wait_for` function cancels the waited task when the timeout
occurs. There are many other cases where a task can be cancelled indirectly.

Don't call :meth:`~Future.set_result` or :meth:`~Future.set_exception` method
of :class:`Future` if the future is cancelled: it would fail with an exception.
For example, write::

    if not fut.cancelled():
        fut.set_result('done')

Don't schedule directly a call to the :meth:`~Future.set_result` or the
:meth:`~Future.set_exception` method of a future with
:meth:`loop.call_soon`: the future can be cancelled before its method
is called.

If you wait for a future, you should check early if the future was cancelled to
avoid useless operations. Example::

    async def slow_operation(fut):
        if fut.cancelled():
            return
        # ... slow computation ...
        await fut
        # ...

The :func:`shield` function can also be used to ignore cancellation.


.. _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 may 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. Prior to suspending the task, the awaiting
chain is checked, and if the chain ends with a future, the running task is
not suspended.

To schedule a callback from a different thread, the
:meth:`loop.call_soon_threadsafe` method should be used. Example::

    loop.call_soon_threadsafe(callback, *args)

Most asyncio objects are not thread safe. You should only worry if you access
objects outside the event loop. For example, to cancel a future, don't call
directly its :meth:`Future.cancel` method, but::

    loop.call_soon_threadsafe(fut.cancel)

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

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

     future = asyncio.run_coroutine_threadsafe(coro_func(), loop)
     result = future.result(timeout)  # Wait for the result with a timeout

The :meth:`loop.run_in_executor` method can be used with a
:class:`concurrent.futures.ThreadPoolExecutor` to execute a callback in
different thread so as not to block the event loop's main thread.

.. seealso::

   The :ref:`Synchronization primitives <asyncio-sync>` section describes ways
   to synchronize tasks.

   The :ref:`Subprocess and threads <asyncio-subprocess-threads>` section lists
   asyncio limitations to run subprocesses from different threads.




.. _asyncio-handle-blocking:

Handle blocking functions correctly
-----------------------------------

Blocking functions should not be called directly. For example, if a function
blocks for 1 second, other tasks are delayed by 1 second which can have an
important impact on reactivity.

For networking and subprocesses, the :mod:`asyncio` module provides high-level
APIs like :ref:`protocols <asyncio-protocol>`.

An executor can be used to run a task in a different thread or even in a
different process, to not block the thread of the event loop. See the
:meth:`loop.run_in_executor` method.

.. seealso::

   The :ref:`Delayed calls <asyncio-delayed-calls>` section details how the
   event loop handles time.


.. _asyncio-logger:

Logging
-------

The :mod:`asyncio` module logs information with the :mod:`logging` module in
the logger ``'asyncio'``.

The default log level for the :mod:`asyncio` module is :py:data:`logging.INFO`.
For those not wanting such verbosity from :mod:`asyncio` the log level can
be changed.  For example, to change the level to :py:data:`logging.WARNING`:

.. code-block:: none

   logging.getLogger('asyncio').setLevel(logging.WARNING)


.. _asyncio-coroutine-not-scheduled:

Detect coroutine objects never scheduled
----------------------------------------

When a coroutine function is called and its result is not passed to
:meth:`asyncio.create_task` the execution of the coroutine object will
never be scheduled which is probably a bug. Using ``asyncio.create_task`` is
preferred to the low level :func:`ensure_future` and :meth:`loop.create_task`
methods. :ref:`Enable the debug mode of asyncio <asyncio-debug-mode>`
to :ref:`log a warning <asyncio-logger>` to detect it.

Example with the bug::

    import asyncio

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

    test()

Output in debug mode::

    Coroutine test() at test.py:3 was never yielded from
    Coroutine object created at (most recent call last):
      File "test.py", line 7, in <module>
        test()

The fix is to call the :meth:`asyncio.create_task` function. Using
``asyncio.create_task`` is preferred to the low level :func:`ensure_future` and
:meth:`loop.create_task` methods.

.. seealso::

   :ref:`Pending task destroyed <asyncio-pending-task-destroyed>`.


Detect exceptions never consumed
--------------------------------

Python usually calls :func:`sys.excepthook` on unhandled exceptions. If
:meth:`Future.set_exception` is called, but the exception is never consumed,
:func:`sys.excepthook` is not called. Instead, :ref:`a log is emitted
<asyncio-logger>` when the future is deleted by the garbage collector, with the
traceback where the exception was raised.

Example of unhandled exception::

    import asyncio

    @asyncio.coroutine
    def bug():
        raise Exception("not consumed")

    loop = asyncio.get_event_loop()
    asyncio.ensure_future(bug())
    loop.run_forever()
    loop.close()

Output::

    Task exception was never retrieved
    future: <Task finished coro=<coro() done, defined at asyncio/coroutines.py:139> exception=Exception('not consumed',)>
    Traceback (most recent call last):
      File "asyncio/tasks.py", line 237, in _step
        result = next(coro)
      File "asyncio/coroutines.py", line 141, in coro
        res = func(*args, **kw)
      File "test.py", line 5, in bug
        raise Exception("not consumed")
    Exception: not consumed

:ref:`Enable the debug mode of asyncio <asyncio-debug-mode>` to get the
traceback where the task was created. 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 test.py:8>
    source_traceback: Object created at (most recent call last):
      File "test.py", line 8, in <module>
        asyncio.ensure_future(bug())
    Traceback (most recent call last):
      File "asyncio/tasks.py", line 237, in _step
        result = next(coro)
      File "asyncio/coroutines.py", line 79, in __next__
        return next(self.gen)
      File "asyncio/coroutines.py", line 141, in coro
        res = func(*args, **kw)
      File "test.py", line 5, in bug
        raise Exception("not consumed")
    Exception: not consumed

There are different options to fix this issue. The first option is to chain the
coroutine in another coroutine and use classic try/except::

    async def handle_exception():
        try:
            await bug()
        except Exception:
            print("exception consumed")

    loop = asyncio.get_event_loop()
    asyncio.ensure_future(handle_exception())
    loop.run_forever()
    loop.close()

Another option is to use the :meth:`asyncio.run` function::

    asyncio.run(bug())

.. seealso::

   The :meth:`Future.exception` method.


Chain coroutines correctly
--------------------------

When a coroutine function calls other coroutine functions and tasks, they
should be chained explicitly with ``await``. Otherwise, the execution is
not guaranteed to be sequential.

Example with different bugs using :func:`asyncio.sleep` to simulate slow
operations::

    import asyncio

    async def create():
        await asyncio.sleep(3.0)
        print("(1) create file")

    async def write():
        await asyncio.sleep(1.0)
        print("(2) write into file")

    async def close():
        print("(3) close file")

    async def test():
        asyncio.ensure_future(create())
        asyncio.ensure_future(write())
        asyncio.ensure_future(close())
        await asyncio.sleep(2.0)
        loop.stop()

    loop = asyncio.get_event_loop()
    asyncio.ensure_future(test())
    loop.run_forever()
    print("Pending tasks at exit: %s" % asyncio.Task.all_tasks(loop))
    loop.close()

Expected output:

.. code-block:: none

    (1) create file
    (2) write into file
    (3) close file
    Pending tasks at exit: set()

Actual output:

.. code-block:: none

    (3) close file
    (2) write into file
    Pending tasks at exit: {<Task pending create() at test.py:7 wait_for=<Future pending cb=[Task._wakeup()]>>}
    Task was destroyed but it is pending!
    task: <Task pending create() done at test.py:5 wait_for=<Future pending cb=[Task._wakeup()]>>

The loop stopped before the ``create()`` finished, ``close()`` has been called
before ``write()``, whereas coroutine functions were called in this order:
``create()``, ``write()``, ``close()``.

To fix the example, tasks must be marked with ``await``::

    async def test():
        await asyncio.ensure_future(create())
        await asyncio.ensure_future(write())
        await asyncio.ensure_future(close())
        await asyncio.sleep(2.0)
        loop.stop()

Or without ``asyncio.ensure_future()``::

    async def test():
        await create()
        await write()
        await close()
        await asyncio.sleep(2.0)
        loop.stop()


.. _asyncio-pending-task-destroyed:

Pending task destroyed
----------------------

If a pending task is destroyed, the execution of its wrapped :ref:`coroutine
<coroutine>` did not complete. It is probably a bug and so a warning is logged.

Example of log:

.. code-block:: none

    Task was destroyed but it is pending!
    task: <Task pending coro=<kill_me() done, defined at test.py:5> wait_for=<Future pending cb=[Task._wakeup()]>>

:ref:`Enable the debug mode of asyncio <asyncio-debug-mode>` to get the
traceback where the task was created. Example of log in debug mode:

.. code-block:: none

    Task was destroyed but it is pending!
    source_traceback: Object created at (most recent call last):
      File "test.py", line 15, in <module>
        task = asyncio.ensure_future(coro, loop=loop)
    task: <Task pending coro=<kill_me() done, defined at test.py:5> wait_for=<Future pending cb=[Task._wakeup()] created at test.py:7> created at test.py:15>


.. seealso::

   :ref:`Detect coroutine objects never scheduled <asyncio-coroutine-not-scheduled>`.

.. _asyncio-close-transports:

Close transports and event loops
--------------------------------

When a transport is no more needed, call its ``close()`` method to release
resources. Event loops must also be closed explicitly.

If a transport or an event loop is not closed explicitly, a
:exc:`ResourceWarning` warning will be emitted in its destructor. By default,
:exc:`ResourceWarning` warnings are ignored. The :ref:`Debug mode of asyncio
<asyncio-debug-mode>` section explains how to display them.