1009 lines
30 KiB
ReStructuredText
1009 lines
30 KiB
ReStructuredText
.. currentmodule:: asyncio
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====================
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Coroutines and Tasks
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====================
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This section outlines high-level asyncio APIs to work with coroutines
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and Tasks.
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.. contents::
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:depth: 1
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:local:
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.. _coroutine:
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Coroutines
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==========
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:term:`Coroutines <coroutine>` declared with the async/await syntax is the
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preferred way of writing asyncio applications. For example, the following
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snippet of code (requires Python 3.7+) prints "hello", waits 1 second,
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and then prints "world"::
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>>> import asyncio
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>>> async def main():
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... print('hello')
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... await asyncio.sleep(1)
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... print('world')
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>>> asyncio.run(main())
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hello
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world
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Note that simply calling a coroutine will not schedule it to
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be executed::
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>>> main()
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<coroutine object main at 0x1053bb7c8>
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To actually run a coroutine, asyncio provides three main mechanisms:
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* The :func:`asyncio.run` function to run the top-level
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entry point "main()" function (see the above example.)
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* Awaiting on a coroutine. The following snippet of code will
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print "hello" after waiting for 1 second, and then print "world"
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after waiting for *another* 2 seconds::
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import asyncio
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import time
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async def say_after(delay, what):
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await asyncio.sleep(delay)
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print(what)
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async def main():
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print(f"started at {time.strftime('%X')}")
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await say_after(1, 'hello')
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await say_after(2, 'world')
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print(f"finished at {time.strftime('%X')}")
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asyncio.run(main())
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Expected output::
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started at 17:13:52
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hello
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world
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finished at 17:13:55
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* The :func:`asyncio.create_task` function to run coroutines
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concurrently as asyncio :class:`Tasks <Task>`.
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Let's modify the above example and run two ``say_after`` coroutines
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*concurrently*::
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async def main():
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task1 = asyncio.create_task(
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say_after(1, 'hello'))
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task2 = asyncio.create_task(
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say_after(2, 'world'))
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print(f"started at {time.strftime('%X')}")
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# Wait until both tasks are completed (should take
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# around 2 seconds.)
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await task1
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await task2
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print(f"finished at {time.strftime('%X')}")
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Note that expected output now shows that the snippet runs
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1 second faster than before::
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started at 17:14:32
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hello
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world
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finished at 17:14:34
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.. _asyncio-awaitables:
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Awaitables
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==========
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We say that an object is an **awaitable** object if it can be used
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in an :keyword:`await` expression. Many asyncio APIs are designed to
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accept awaitables.
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There are three main types of *awaitable* objects:
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**coroutines**, **Tasks**, and **Futures**.
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.. rubric:: Coroutines
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Python coroutines are *awaitables* and therefore can be awaited from
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other coroutines::
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import asyncio
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async def nested():
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return 42
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async def main():
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# Nothing happens if we just call "nested()".
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# A coroutine object is created but not awaited,
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# so it *won't run at all*.
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nested()
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# Let's do it differently now and await it:
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print(await nested()) # will print "42".
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asyncio.run(main())
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.. important::
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In this documentation the term "coroutine" can be used for
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two closely related concepts:
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* a *coroutine function*: an :keyword:`async def` function;
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* a *coroutine object*: an object returned by calling a
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*coroutine function*.
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asyncio also supports legacy :ref:`generator-based
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<asyncio_generator_based_coro>` coroutines.
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.. rubric:: Tasks
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*Tasks* are used to schedule coroutines *concurrently*.
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When a coroutine is wrapped into a *Task* with functions like
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:func:`asyncio.create_task` the coroutine is automatically
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scheduled to run soon::
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import asyncio
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async def nested():
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return 42
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async def main():
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# Schedule nested() to run soon concurrently
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# with "main()".
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task = asyncio.create_task(nested())
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# "task" can now be used to cancel "nested()", or
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# can simply be awaited to wait until it is complete:
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await task
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asyncio.run(main())
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.. rubric:: Futures
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A :class:`Future` is a special **low-level** awaitable object that
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represents an **eventual result** of an asynchronous operation.
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When a Future object is *awaited* it means that the coroutine will
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wait until the Future is resolved in some other place.
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Future objects in asyncio are needed to allow callback-based code
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to be used with async/await.
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Normally **there is no need** to create Future objects at the
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application level code.
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Future objects, sometimes exposed by libraries and some asyncio
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APIs, can be awaited::
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async def main():
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await function_that_returns_a_future_object()
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# this is also valid:
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await asyncio.gather(
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function_that_returns_a_future_object(),
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some_python_coroutine()
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)
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A good example of a low-level function that returns a Future object
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is :meth:`loop.run_in_executor`.
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Running an asyncio Program
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==========================
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.. function:: run(coro, \*, debug=False)
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Execute the :term:`coroutine` *coro* and return the result.
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This function runs the passed coroutine, taking care of
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managing the asyncio event loop, *finalizing asynchronous
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generators*, and closing the threadpool.
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This function cannot be called when another asyncio event loop is
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running in the same thread.
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If *debug* is ``True``, the event loop will be run in debug mode.
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This function always creates a new event loop and closes it at
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the end. It should be used as a main entry point for asyncio
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programs, and should ideally only be called once.
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Example::
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async def main():
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await asyncio.sleep(1)
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print('hello')
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asyncio.run(main())
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.. versionadded:: 3.7
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.. versionchanged:: 3.9
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Updated to use :meth:`loop.shutdown_default_executor`.
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.. note::
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The source code for ``asyncio.run()`` can be found in
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:source:`Lib/asyncio/runners.py`.
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Creating Tasks
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==============
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.. function:: create_task(coro, \*, name=None)
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Wrap the *coro* :ref:`coroutine <coroutine>` into a :class:`Task`
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and schedule its execution. Return the Task object.
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If *name* is not ``None``, it is set as the name of the task using
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:meth:`Task.set_name`.
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The task is executed in the loop returned by :func:`get_running_loop`,
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:exc:`RuntimeError` is raised if there is no running loop in
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current thread.
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This function has been **added in Python 3.7**. Prior to
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Python 3.7, the low-level :func:`asyncio.ensure_future` function
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can be used instead::
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async def coro():
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...
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# In Python 3.7+
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task = asyncio.create_task(coro())
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...
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# This works in all Python versions but is less readable
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task = asyncio.ensure_future(coro())
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...
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.. versionadded:: 3.7
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.. versionchanged:: 3.8
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Added the ``name`` parameter.
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Sleeping
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========
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.. coroutinefunction:: sleep(delay, result=None)
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Block for *delay* seconds.
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If *result* is provided, it is returned to the caller
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when the coroutine completes.
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``sleep()`` always suspends the current task, allowing other tasks
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to run.
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.. _asyncio_example_sleep:
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Example of coroutine displaying the current date every second
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for 5 seconds::
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import asyncio
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import datetime
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async def display_date():
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loop = asyncio.get_running_loop()
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end_time = loop.time() + 5.0
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while True:
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print(datetime.datetime.now())
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if (loop.time() + 1.0) >= end_time:
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break
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await asyncio.sleep(1)
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asyncio.run(display_date())
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Running Tasks Concurrently
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==========================
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.. awaitablefunction:: gather(\*aws, return_exceptions=False)
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Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
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sequence *concurrently*.
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If any awaitable in *aws* is a coroutine, it is automatically
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scheduled as a Task.
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If all awaitables are completed successfully, the result is an
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aggregate list of returned values. The order of result values
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corresponds to the order of awaitables in *aws*.
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If *return_exceptions* is ``False`` (default), the first
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raised exception is immediately propagated to the task that
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awaits on ``gather()``. Other awaitables in the *aws* sequence
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**won't be cancelled** and will continue to run.
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If *return_exceptions* is ``True``, exceptions are treated the
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same as successful results, and aggregated in the result list.
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If ``gather()`` is *cancelled*, all submitted awaitables
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(that have not completed yet) are also *cancelled*.
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If any Task or Future from the *aws* sequence is *cancelled*, it is
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treated as if it raised :exc:`CancelledError` -- the ``gather()``
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call is **not** cancelled in this case. This is to prevent the
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cancellation of one submitted Task/Future to cause other
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Tasks/Futures to be cancelled.
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.. _asyncio_example_gather:
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Example::
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import asyncio
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async def factorial(name, number):
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f = 1
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for i in range(2, number + 1):
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print(f"Task {name}: Compute factorial({i})...")
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await asyncio.sleep(1)
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f *= i
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print(f"Task {name}: factorial({number}) = {f}")
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async def main():
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# Schedule three calls *concurrently*:
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await asyncio.gather(
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factorial("A", 2),
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factorial("B", 3),
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factorial("C", 4),
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)
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asyncio.run(main())
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# Expected output:
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#
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# Task A: Compute factorial(2)...
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# Task B: Compute factorial(2)...
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# Task C: Compute factorial(2)...
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# Task A: factorial(2) = 2
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# Task B: Compute factorial(3)...
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# Task C: Compute factorial(3)...
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# Task B: factorial(3) = 6
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# Task C: Compute factorial(4)...
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# Task C: factorial(4) = 24
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.. note::
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If *return_exceptions* is False, cancelling gather() after it
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has been marked done won't cancel any submitted awaitables.
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For instance, gather can be marked done after propagating an
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exception to the caller, therefore, calling ``gather.cancel()``
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after catching an exception (raised by one of the awaitables) from
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gather won't cancel any other awaitables.
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.. versionchanged:: 3.7
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If the *gather* itself is cancelled, the cancellation is
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propagated regardless of *return_exceptions*.
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Shielding From Cancellation
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===========================
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.. awaitablefunction:: shield(aw)
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Protect an :ref:`awaitable object <asyncio-awaitables>`
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from being :meth:`cancelled <Task.cancel>`.
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If *aw* is a coroutine it is automatically scheduled as a Task.
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The statement::
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res = await shield(something())
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is equivalent to::
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res = await something()
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*except* that if the coroutine containing it is cancelled, the
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Task running in ``something()`` is not cancelled. From the point
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of view of ``something()``, the cancellation did not happen.
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Although its caller is still cancelled, so the "await" expression
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still raises a :exc:`CancelledError`.
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If ``something()`` is cancelled by other means (i.e. from within
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itself) that would also cancel ``shield()``.
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If it is desired to completely ignore cancellation (not recommended)
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the ``shield()`` function should be combined with a try/except
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clause, as follows::
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try:
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res = await shield(something())
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except CancelledError:
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res = None
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Timeouts
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========
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.. coroutinefunction:: wait_for(aw, timeout)
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Wait for the *aw* :ref:`awaitable <asyncio-awaitables>`
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to complete with a timeout.
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If *aw* is a coroutine it is automatically scheduled as a Task.
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*timeout* can either be ``None`` or a float or int number of seconds
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to wait for. If *timeout* is ``None``, block until the future
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completes.
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If a timeout occurs, it cancels the task and raises
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:exc:`asyncio.TimeoutError`.
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To avoid the task :meth:`cancellation <Task.cancel>`,
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wrap it in :func:`shield`.
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The function will wait until the future is actually cancelled,
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so the total wait time may exceed the *timeout*. If an exception
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happens during cancellation, it is propagated.
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If the wait is cancelled, the future *aw* is also cancelled.
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.. _asyncio_example_waitfor:
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Example::
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async def eternity():
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# Sleep for one hour
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await asyncio.sleep(3600)
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print('yay!')
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async def main():
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# Wait for at most 1 second
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try:
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await asyncio.wait_for(eternity(), timeout=1.0)
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except asyncio.TimeoutError:
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print('timeout!')
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asyncio.run(main())
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# Expected output:
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#
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# timeout!
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.. versionchanged:: 3.7
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When *aw* is cancelled due to a timeout, ``wait_for`` waits
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for *aw* to be cancelled. Previously, it raised
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:exc:`asyncio.TimeoutError` immediately.
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Waiting Primitives
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==================
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.. coroutinefunction:: wait(aws, \*, timeout=None, return_when=ALL_COMPLETED)
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Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
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iterable concurrently and block until the condition specified
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by *return_when*.
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The *aws* iterable must not be empty.
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Returns two sets of Tasks/Futures: ``(done, pending)``.
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Usage::
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done, pending = await asyncio.wait(aws)
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*timeout* (a float or int), if specified, can be used to control
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the maximum number of seconds to wait before returning.
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Note that this function does not raise :exc:`asyncio.TimeoutError`.
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Futures or Tasks that aren't done when the timeout occurs are simply
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returned in the second set.
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*return_when* indicates when this function should return. It must
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be one of the following constants:
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.. tabularcolumns:: |l|L|
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+-----------------------------+----------------------------------------+
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| Constant | Description |
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+=============================+========================================+
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| :const:`FIRST_COMPLETED` | The function will return when any |
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| | future finishes or is cancelled. |
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+-----------------------------+----------------------------------------+
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| :const:`FIRST_EXCEPTION` | The function will return when any |
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| | future finishes by raising an |
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| | exception. If no future raises an |
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| | exception then it is equivalent to |
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| | :const:`ALL_COMPLETED`. |
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+-----------------------------+----------------------------------------+
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| :const:`ALL_COMPLETED` | The function will return when all |
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| | futures finish or are cancelled. |
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+-----------------------------+----------------------------------------+
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Unlike :func:`~asyncio.wait_for`, ``wait()`` does not cancel the
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futures when a timeout occurs.
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.. deprecated:: 3.8
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If any awaitable in *aws* is a coroutine, it is automatically
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scheduled as a Task. Passing coroutines objects to
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``wait()`` directly is deprecated as it leads to
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:ref:`confusing behavior <asyncio_example_wait_coroutine>`.
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.. _asyncio_example_wait_coroutine:
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.. note::
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``wait()`` schedules coroutines as Tasks automatically and later
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returns those implicitly created Task objects in ``(done, pending)``
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sets. Therefore the following code won't work as expected::
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async def foo():
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return 42
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coro = foo()
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done, pending = await asyncio.wait({coro})
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if coro in done:
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# This branch will never be run!
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Here is how the above snippet can be fixed::
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async def foo():
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return 42
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task = asyncio.create_task(foo())
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done, pending = await asyncio.wait({task})
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if task in done:
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# Everything will work as expected now.
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.. deprecated-removed:: 3.8 3.11
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Passing coroutine objects to ``wait()`` directly is
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deprecated.
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.. function:: as_completed(aws, \*, timeout=None)
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Run :ref:`awaitable objects <asyncio-awaitables>` in the *aws*
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iterable concurrently. Return an iterator of coroutines.
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Each coroutine returned can be awaited to get the earliest next
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result from the iterable of the remaining awaitables.
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Raises :exc:`asyncio.TimeoutError` if the timeout occurs before
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all Futures are done.
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Example::
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for coro in as_completed(aws):
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earliest_result = await coro
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# ...
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Running in Threads
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==================
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.. coroutinefunction:: to_thread(func, /, \*args, \*\*kwargs)
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Asynchronously run function *func* in a separate thread.
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Any \*args and \*\*kwargs supplied for this function are directly passed
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to *func*. Also, the current :class:`contextvars.Context` is propagated,
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allowing context variables from the event loop thread to be accessed in the
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separate thread.
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Return a coroutine that can be awaited to get the eventual result of *func*.
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This coroutine function is primarily intended to be used for executing
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IO-bound functions/methods that would otherwise block the event loop if
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they were ran in the main thread. For example::
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def blocking_io():
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print(f"start blocking_io at {time.strftime('%X')}")
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# Note that time.sleep() can be replaced with any blocking
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# IO-bound operation, such as file operations.
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time.sleep(1)
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print(f"blocking_io complete at {time.strftime('%X')}")
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async def main():
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print(f"started main at {time.strftime('%X')}")
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await asyncio.gather(
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asyncio.to_thread(blocking_io),
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asyncio.sleep(1))
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print(f"finished main at {time.strftime('%X')}")
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asyncio.run(main())
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# Expected output:
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#
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# started main at 19:50:53
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# start blocking_io at 19:50:53
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# blocking_io complete at 19:50:54
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# finished main at 19:50:54
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Directly calling `blocking_io()` in any coroutine would block the event loop
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for its duration, resulting in an additional 1 second of run time. Instead,
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by using `asyncio.to_thread()`, we can run it in a separate thread without
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blocking the event loop.
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.. note::
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Due to the :term:`GIL`, `asyncio.to_thread()` can typically only be used
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to make IO-bound functions non-blocking. However, for extension modules
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that release the GIL or alternative Python implementations that don't
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have one, `asyncio.to_thread()` can also be used for CPU-bound functions.
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.. versionadded:: 3.9
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Scheduling From Other Threads
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=============================
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.. function:: run_coroutine_threadsafe(coro, loop)
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Submit a coroutine to the given event loop. Thread-safe.
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Return a :class:`concurrent.futures.Future` to wait for the result
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from another OS thread.
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This function is meant to be called from a different OS thread
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than the one where the event loop is running. Example::
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# Create a coroutine
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coro = asyncio.sleep(1, result=3)
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# Submit the coroutine to a given loop
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future = asyncio.run_coroutine_threadsafe(coro, loop)
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# Wait for the result with an optional timeout argument
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assert future.result(timeout) == 3
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If an exception is raised in the coroutine, the returned Future
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will be notified. It can also be used to cancel the task in
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the event loop::
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try:
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result = future.result(timeout)
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except asyncio.TimeoutError:
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print('The coroutine took too long, cancelling the task...')
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future.cancel()
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except Exception as exc:
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print(f'The coroutine raised an exception: {exc!r}')
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else:
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print(f'The coroutine returned: {result!r}')
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See the :ref:`concurrency and multithreading <asyncio-multithreading>`
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section of the documentation.
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Unlike other asyncio functions this function requires the *loop*
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argument to be passed explicitly.
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.. versionadded:: 3.5.1
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Introspection
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=============
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.. function:: current_task(loop=None)
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Return the currently running :class:`Task` instance, or ``None`` if
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no task is running.
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If *loop* is ``None`` :func:`get_running_loop` is used to get
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the current loop.
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.. versionadded:: 3.7
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.. function:: all_tasks(loop=None)
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Return a set of not yet finished :class:`Task` objects run by
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the loop.
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If *loop* is ``None``, :func:`get_running_loop` is used for getting
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current loop.
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.. versionadded:: 3.7
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Task Object
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===========
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.. class:: Task(coro, \*, loop=None, name=None)
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A :class:`Future-like <Future>` object that runs a Python
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:ref:`coroutine <coroutine>`. Not thread-safe.
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Tasks are used to run coroutines in event loops.
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If a coroutine awaits on a Future, the Task suspends
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the execution of the coroutine and waits for the completion
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of the Future. When the Future is *done*, the execution of
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the wrapped coroutine resumes.
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Event loops use cooperative scheduling: an event loop runs
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one Task at a time. While a Task awaits for the completion of a
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Future, the event loop runs other Tasks, callbacks, or performs
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IO operations.
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Use the high-level :func:`asyncio.create_task` function to create
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Tasks, or the low-level :meth:`loop.create_task` or
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:func:`ensure_future` functions. Manual instantiation of Tasks
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is discouraged.
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To cancel a running Task use the :meth:`cancel` method. Calling it
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will cause the Task to throw a :exc:`CancelledError` exception into
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the wrapped coroutine. If a coroutine is awaiting on a Future
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object during cancellation, the Future object will be cancelled.
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:meth:`cancelled` can be used to check if the Task was cancelled.
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The method returns ``True`` if the wrapped coroutine did not
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suppress the :exc:`CancelledError` exception and was actually
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cancelled.
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:class:`asyncio.Task` inherits from :class:`Future` all of its
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APIs except :meth:`Future.set_result` and
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:meth:`Future.set_exception`.
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Tasks support the :mod:`contextvars` module. When a Task
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is created it copies the current context and later runs its
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coroutine in the copied context.
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.. versionchanged:: 3.7
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Added support for the :mod:`contextvars` module.
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.. versionchanged:: 3.8
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Added the ``name`` parameter.
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.. deprecated-removed:: 3.8 3.10
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The *loop* parameter.
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.. method:: cancel(msg=None)
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Request the Task to be cancelled.
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This arranges for a :exc:`CancelledError` exception to be thrown
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into the wrapped coroutine on the next cycle of the event loop.
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The coroutine then has a chance to clean up or even deny the
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request by suppressing the exception with a :keyword:`try` ...
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... ``except CancelledError`` ... :keyword:`finally` block.
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Therefore, unlike :meth:`Future.cancel`, :meth:`Task.cancel` does
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not guarantee that the Task will be cancelled, although
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suppressing cancellation completely is not common and is actively
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discouraged.
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.. versionchanged:: 3.9
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Added the ``msg`` parameter.
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.. _asyncio_example_task_cancel:
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The following example illustrates how coroutines can intercept
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the cancellation request::
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async def cancel_me():
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print('cancel_me(): before sleep')
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try:
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# Wait for 1 hour
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await asyncio.sleep(3600)
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except asyncio.CancelledError:
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print('cancel_me(): cancel sleep')
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raise
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finally:
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print('cancel_me(): after sleep')
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async def main():
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# Create a "cancel_me" Task
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task = asyncio.create_task(cancel_me())
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# Wait for 1 second
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await asyncio.sleep(1)
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task.cancel()
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try:
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await task
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except asyncio.CancelledError:
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print("main(): cancel_me is cancelled now")
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asyncio.run(main())
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# Expected output:
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#
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# cancel_me(): before sleep
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# cancel_me(): cancel sleep
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# cancel_me(): after sleep
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# main(): cancel_me is cancelled now
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.. method:: cancelled()
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Return ``True`` if the Task is *cancelled*.
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The Task is *cancelled* when the cancellation was requested with
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:meth:`cancel` and the wrapped coroutine propagated the
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:exc:`CancelledError` exception thrown into it.
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.. method:: done()
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Return ``True`` if the Task is *done*.
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A Task is *done* when the wrapped coroutine either returned
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a value, raised an exception, or the Task was cancelled.
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.. method:: result()
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Return the result of the Task.
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If the Task is *done*, the result of the wrapped coroutine
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is returned (or if the coroutine raised an exception, that
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exception is re-raised.)
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If the Task has been *cancelled*, this method raises
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a :exc:`CancelledError` exception.
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If the Task's result isn't yet available, this method raises
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a :exc:`InvalidStateError` exception.
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.. method:: exception()
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Return the exception of the Task.
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If the wrapped coroutine raised an exception that exception
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is returned. If the wrapped coroutine returned normally
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this method returns ``None``.
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If the Task has been *cancelled*, this method raises a
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:exc:`CancelledError` exception.
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If the Task isn't *done* yet, this method raises an
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:exc:`InvalidStateError` exception.
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.. method:: add_done_callback(callback, *, context=None)
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Add a callback to be run when the Task is *done*.
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This method should only be used in low-level callback-based code.
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See the documentation of :meth:`Future.add_done_callback`
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for more details.
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.. method:: remove_done_callback(callback)
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Remove *callback* from the callbacks list.
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This method should only be used in low-level callback-based code.
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See the documentation of :meth:`Future.remove_done_callback`
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for more details.
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.. method:: get_stack(\*, limit=None)
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Return the list of stack frames for this Task.
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If the wrapped coroutine is not done, this returns the stack
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where it is suspended. If the coroutine has completed
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successfully or was cancelled, this returns an empty list.
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If the coroutine was terminated by an exception, this returns
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the list of traceback frames.
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The frames are always ordered from oldest to newest.
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Only one stack frame is returned for a suspended coroutine.
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The optional *limit* argument sets the maximum number of frames
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to return; by default all available frames are returned.
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The ordering of the returned list differs depending on whether
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a stack or a traceback is returned: the newest frames of a
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stack are returned, but the oldest frames of a traceback are
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returned. (This matches the behavior of the traceback module.)
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.. method:: print_stack(\*, limit=None, file=None)
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Print the stack or traceback for this Task.
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This produces output similar to that of the traceback module
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for the frames retrieved by :meth:`get_stack`.
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The *limit* argument is passed to :meth:`get_stack` directly.
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The *file* argument is an I/O stream to which the output
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is written; by default output is written to :data:`sys.stderr`.
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.. method:: get_coro()
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Return the coroutine object wrapped by the :class:`Task`.
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.. versionadded:: 3.8
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.. method:: get_name()
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Return the name of the Task.
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If no name has been explicitly assigned to the Task, the default
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asyncio Task implementation generates a default name during
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instantiation.
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.. versionadded:: 3.8
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.. method:: set_name(value)
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Set the name of the Task.
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The *value* argument can be any object, which is then
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converted to a string.
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In the default Task implementation, the name will be visible
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in the :func:`repr` output of a task object.
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.. versionadded:: 3.8
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.. _asyncio_generator_based_coro:
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Generator-based Coroutines
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==========================
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.. note::
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Support for generator-based coroutines is **deprecated** and
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is scheduled for removal in Python 3.10.
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Generator-based coroutines predate async/await syntax. They are
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Python generators that use ``yield from`` expressions to await
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on Futures and other coroutines.
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Generator-based coroutines should be decorated with
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:func:`@asyncio.coroutine <asyncio.coroutine>`, although this is not
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enforced.
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.. decorator:: coroutine
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Decorator to mark generator-based coroutines.
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This decorator enables legacy generator-based coroutines to be
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compatible with async/await code::
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@asyncio.coroutine
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def old_style_coroutine():
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yield from asyncio.sleep(1)
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async def main():
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await old_style_coroutine()
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This decorator should not be used for :keyword:`async def`
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coroutines.
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.. deprecated-removed:: 3.8 3.10
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Use :keyword:`async def` instead.
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.. function:: iscoroutine(obj)
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Return ``True`` if *obj* is a :ref:`coroutine object <coroutine>`.
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This method is different from :func:`inspect.iscoroutine` because
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it returns ``True`` for generator-based coroutines.
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.. function:: iscoroutinefunction(func)
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Return ``True`` if *func* is a :ref:`coroutine function
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<coroutine>`.
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This method is different from :func:`inspect.iscoroutinefunction`
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because it returns ``True`` for generator-based coroutine functions
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decorated with :func:`@coroutine <coroutine>`.
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