417 lines
13 KiB
ReStructuredText
417 lines
13 KiB
ReStructuredText
.. 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.
|