417 lines
13 KiB
ReStructuredText
417 lines
13 KiB
ReStructuredText
.. currentmodule:: asyncio
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.. _asyncio-dev:
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Develop with asyncio
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====================
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Asynchronous programming is different than classical "sequential" programming.
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This page lists common traps and explains how to avoid them.
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.. _asyncio-debug-mode:
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Debug mode of asyncio
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---------------------
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The implementation of :mod:`asyncio` has been written for performance.
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In order to ease the development of asynchronous code, you may wish to
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enable *debug mode*.
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To enable all debug checks for an application:
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* Enable the asyncio debug mode globally by setting the environment variable
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:envvar:`PYTHONASYNCIODEBUG` to ``1``, using ``-X dev`` command line option
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(see the :option:`-X` option), or by calling
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:meth:`loop.set_debug`.
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* Set the log level of the :ref:`asyncio logger <asyncio-logger>` to
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:py:data:`logging.DEBUG`. For example, call
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``logging.basicConfig(level=logging.DEBUG)`` at startup.
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* Configure the :mod:`warnings` module to display :exc:`ResourceWarning`
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warnings. For example, use the ``-Wdefault`` command line option of Python to
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display them.
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Examples debug checks:
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* Log :ref:`coroutines defined but never "yielded from"
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<asyncio-coroutine-not-scheduled>`
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* :meth:`loop.call_soon` and :meth:`loop.call_at` methods
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raise an exception if they are called from the wrong thread.
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* Log the execution time of the selector
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* Log callbacks taking more than 100 ms to be executed. The
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:attr:`loop.slow_callback_duration` attribute is the minimum
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duration in seconds of "slow" callbacks.
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* :exc:`ResourceWarning` warnings are emitted when transports and event loops
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are :ref:`not closed explicitly <asyncio-close-transports>`.
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.. versionchanged:: 3.7
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The new ``-X dev`` command line option can now also be used to enable
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the debug mode.
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.. seealso::
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The :meth:`loop.set_debug` method and the :ref:`asyncio logger
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<asyncio-logger>`.
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Cancellation
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------------
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Cancellation of tasks is not common in classic programming. In asynchronous
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programming, not only is it something common, but you have to prepare your
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code to handle it.
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Futures and tasks can be cancelled explicitly with their :meth:`Future.cancel`
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method. The :func:`wait_for` function cancels the waited task when the timeout
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occurs. There are many other cases where a task can be cancelled indirectly.
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Don't call :meth:`~Future.set_result` or :meth:`~Future.set_exception` method
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of :class:`Future` if the future is cancelled: it would fail with an exception.
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For example, write::
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if not fut.cancelled():
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fut.set_result('done')
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Don't schedule directly a call to the :meth:`~Future.set_result` or the
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:meth:`~Future.set_exception` method of a future with
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:meth:`loop.call_soon`: the future can be cancelled before its method
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is called.
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If you wait for a future, you should check early if the future was cancelled to
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avoid useless operations. Example::
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async def slow_operation(fut):
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if fut.cancelled():
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return
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# ... slow computation ...
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await fut
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# ...
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The :func:`shield` function can also be used to ignore cancellation.
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.. _asyncio-multithreading:
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Concurrency and multithreading
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------------------------------
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An event loop runs in a thread (typically the main thread) and executes
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all callbacks and tasks in its thread. While a task is running in the
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event loop, no other tasks may run in the same thread. When a task
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executes an ``await`` expression, the running task gets suspended, and the
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event loop executes the next task. Prior to suspending the task, the awaiting
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chain is checked, and if the chain ends with a future, the running task is
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not suspended.
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To schedule a callback from a different thread, the
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:meth:`loop.call_soon_threadsafe` method should be used. Example::
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loop.call_soon_threadsafe(callback, *args)
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Most asyncio objects are not thread safe. You should only worry if you access
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objects outside the event loop. For example, to cancel a future, don't call
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directly its :meth:`Future.cancel` method, but::
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loop.call_soon_threadsafe(fut.cancel)
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To handle signals and to execute subprocesses, the event loop must be run in
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the main thread.
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To schedule a coroutine object from a different thread, the
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:func:`run_coroutine_threadsafe` function should be used. It returns a
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:class:`concurrent.futures.Future` to access the result::
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future = asyncio.run_coroutine_threadsafe(coro_func(), loop)
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result = future.result(timeout) # Wait for the result with a timeout
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The :meth:`loop.run_in_executor` method can be used with a
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:class:`concurrent.futures.ThreadPoolExecutor` to execute a callback in
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different thread so as not to block the event loop's main thread.
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.. seealso::
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The :ref:`Synchronization primitives <asyncio-sync>` section describes ways
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to synchronize tasks.
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The :ref:`Subprocess and threads <asyncio-subprocess-threads>` section lists
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asyncio limitations to run subprocesses from different threads.
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.. _asyncio-handle-blocking:
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Handle blocking functions correctly
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-----------------------------------
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Blocking functions should not be called directly. For example, if a function
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blocks for 1 second, other tasks are delayed by 1 second which can have an
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important impact on reactivity.
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For networking and subprocesses, the :mod:`asyncio` module provides high-level
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APIs like :ref:`protocols <asyncio-protocol>`.
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An executor can be used to run a task in a different thread or even in a
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different process, to not block the thread of the event loop. See the
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:meth:`loop.run_in_executor` method.
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.. seealso::
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The :ref:`Delayed calls <asyncio-delayed-calls>` section details how the
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event loop handles time.
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.. _asyncio-logger:
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Logging
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-------
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The :mod:`asyncio` module logs information with the :mod:`logging` module in
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the logger ``'asyncio'``.
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The default log level for the :mod:`asyncio` module is :py:data:`logging.INFO`.
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For those not wanting such verbosity from :mod:`asyncio` the log level can
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be changed. For example, to change the level to :py:data:`logging.WARNING`:
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.. code-block:: none
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logging.getLogger('asyncio').setLevel(logging.WARNING)
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.. _asyncio-coroutine-not-scheduled:
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Detect coroutine objects never scheduled
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----------------------------------------
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When a coroutine function is called and its result is not passed to
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:meth:`asyncio.create_task` the execution of the coroutine object will
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never be scheduled which is probably a bug. Using ``asyncio.create_task`` is
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preferred to the low level :func:`ensure_future` and :meth:`loop.create_task`
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methods. :ref:`Enable the debug mode of asyncio <asyncio-debug-mode>`
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to :ref:`log a warning <asyncio-logger>` to detect it.
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Example with the bug::
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import asyncio
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async def test():
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print("never scheduled")
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test()
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Output in debug mode::
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Coroutine test() at test.py:3 was never yielded from
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Coroutine object created at (most recent call last):
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File "test.py", line 7, in <module>
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test()
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The fix is to call the :meth:`asyncio.create_task` function. Using
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``asyncio.create_task`` is preferred to the low level :func:`ensure_future` and
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:meth:`loop.create_task` methods.
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.. seealso::
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:ref:`Pending task destroyed <asyncio-pending-task-destroyed>`.
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Detect exceptions never consumed
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--------------------------------
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Python usually calls :func:`sys.excepthook` on unhandled exceptions. If
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:meth:`Future.set_exception` is called, but the exception is never consumed,
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:func:`sys.excepthook` is not called. Instead, :ref:`a log is emitted
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<asyncio-logger>` when the future is deleted by the garbage collector, with the
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traceback where the exception was raised.
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Example of unhandled exception::
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import asyncio
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@asyncio.coroutine
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def bug():
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raise Exception("not consumed")
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loop = asyncio.get_event_loop()
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asyncio.ensure_future(bug())
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loop.run_forever()
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loop.close()
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Output::
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Task exception was never retrieved
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future: <Task finished coro=<coro() done, defined at asyncio/coroutines.py:139> exception=Exception('not consumed',)>
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Traceback (most recent call last):
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File "asyncio/tasks.py", line 237, in _step
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result = next(coro)
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File "asyncio/coroutines.py", line 141, in coro
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res = func(*args, **kw)
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File "test.py", line 5, in bug
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raise Exception("not consumed")
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Exception: not consumed
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:ref:`Enable the debug mode of asyncio <asyncio-debug-mode>` to get the
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traceback where the task was created. Output in debug mode::
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Task exception was never retrieved
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future: <Task finished coro=<bug() done, defined at test.py:3> exception=Exception('not consumed',) created at test.py:8>
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source_traceback: Object created at (most recent call last):
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File "test.py", line 8, in <module>
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asyncio.ensure_future(bug())
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Traceback (most recent call last):
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File "asyncio/tasks.py", line 237, in _step
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result = next(coro)
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File "asyncio/coroutines.py", line 79, in __next__
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return next(self.gen)
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File "asyncio/coroutines.py", line 141, in coro
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res = func(*args, **kw)
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File "test.py", line 5, in bug
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raise Exception("not consumed")
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Exception: not consumed
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There are different options to fix this issue. The first option is to chain the
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coroutine in another coroutine and use classic try/except::
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async def handle_exception():
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try:
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await bug()
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except Exception:
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print("exception consumed")
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loop = asyncio.get_event_loop()
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asyncio.ensure_future(handle_exception())
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loop.run_forever()
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loop.close()
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Another option is to use the :meth:`asyncio.run` function::
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asyncio.run(bug())
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.. seealso::
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The :meth:`Future.exception` method.
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Chain coroutines correctly
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--------------------------
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When a coroutine function calls other coroutine functions and tasks, they
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should be chained explicitly with ``await``. Otherwise, the execution is
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not guaranteed to be sequential.
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Example with different bugs using :func:`asyncio.sleep` to simulate slow
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operations::
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import asyncio
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async def create():
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await asyncio.sleep(3.0)
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print("(1) create file")
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async def write():
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await asyncio.sleep(1.0)
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print("(2) write into file")
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async def close():
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print("(3) close file")
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async def test():
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asyncio.ensure_future(create())
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asyncio.ensure_future(write())
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asyncio.ensure_future(close())
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await asyncio.sleep(2.0)
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loop.stop()
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loop = asyncio.get_event_loop()
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asyncio.ensure_future(test())
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loop.run_forever()
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print("Pending tasks at exit: %s" % asyncio.Task.all_tasks(loop))
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loop.close()
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Expected output:
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.. code-block:: none
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(1) create file
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(2) write into file
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(3) close file
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Pending tasks at exit: set()
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Actual output:
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.. code-block:: none
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(3) close file
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(2) write into file
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Pending tasks at exit: {<Task pending create() at test.py:7 wait_for=<Future pending cb=[Task._wakeup()]>>}
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Task was destroyed but it is pending!
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task: <Task pending create() done at test.py:5 wait_for=<Future pending cb=[Task._wakeup()]>>
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The loop stopped before the ``create()`` finished, ``close()`` has been called
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before ``write()``, whereas coroutine functions were called in this order:
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``create()``, ``write()``, ``close()``.
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To fix the example, tasks must be marked with ``await``::
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async def test():
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await asyncio.ensure_future(create())
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await asyncio.ensure_future(write())
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await asyncio.ensure_future(close())
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await asyncio.sleep(2.0)
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loop.stop()
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Or without ``asyncio.ensure_future()``::
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async def test():
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await create()
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await write()
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await close()
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await asyncio.sleep(2.0)
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loop.stop()
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.. _asyncio-pending-task-destroyed:
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Pending task destroyed
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----------------------
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If a pending task is destroyed, the execution of its wrapped :ref:`coroutine
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<coroutine>` did not complete. It is probably a bug and so a warning is logged.
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Example of log:
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.. code-block:: none
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Task was destroyed but it is pending!
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task: <Task pending coro=<kill_me() done, defined at test.py:5> wait_for=<Future pending cb=[Task._wakeup()]>>
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:ref:`Enable the debug mode of asyncio <asyncio-debug-mode>` to get the
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traceback where the task was created. Example of log in debug mode:
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.. code-block:: none
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Task was destroyed but it is pending!
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source_traceback: Object created at (most recent call last):
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File "test.py", line 15, in <module>
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task = asyncio.ensure_future(coro, loop=loop)
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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>
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.. seealso::
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:ref:`Detect coroutine objects never scheduled <asyncio-coroutine-not-scheduled>`.
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.. _asyncio-close-transports:
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Close transports and event loops
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--------------------------------
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When a transport is no longer needed, call its ``close()`` method to release
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resources. Event loops must also be closed explicitly.
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If a transport or an event loop is not closed explicitly, a
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:exc:`ResourceWarning` warning will be emitted in its destructor. By default,
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:exc:`ResourceWarning` warnings are ignored. The :ref:`Debug mode of asyncio
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<asyncio-debug-mode>` section explains how to display them.
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