cpython/Doc/library/asyncio-dev.rst

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.. currentmodule:: asyncio
Develop with asyncio
====================
Asynchronous programming is different than classical "sequential" programming.
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This page lists common traps and explains how to avoid them.
.. _asyncio-multithreading:
Concurrency and multithreading
------------------------------
An event loop runs in a thread and executes all callbacks and tasks in the same
thread. While a task is running in the event loop, no other task is running in
the same thread. But when the task uses ``yield from``, the task is suspended
and the event loop executes the next task.
To schedule a callback from a different thread, the
:meth:`BaseEventLoop.call_soon_threadsafe` method should be used. Example to
schedule a coroutine from a different thread::
loop.call_soon_threadsafe(asyncio.async, coro_func())
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.
The :meth:`BaseEventLoop.run_in_executor` method can be used with a thread pool
executor to execute a callback in different thread to not block the thread of
the event loop.
.. seealso::
See the :ref:`Synchronization primitives <asyncio-sync>` section to
synchronize tasks.
.. _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:`BaseEventLoop.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'``.
.. _asyncio-coroutine-not-scheduled:
Detect coroutine objects never scheduled
----------------------------------------
When a coroutine function is called but not passed to :func:`async` or to the
:class:`Task` constructor, it is not scheduled and it is probably a bug.
To detect such bug, set :data:`asyncio.tasks._DEBUG` to ``True``. When the
coroutine object is destroyed by the garbage collector, a log will be emitted
with the traceback where the coroutine function was called. See the
:ref:`asyncio logger <asyncio-logger>`.
The debug flag changes the behaviour of the :func:`coroutine` decorator. The
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debug flag value is only used when then coroutine function is defined, not when
it is called. Coroutine functions defined before the debug flag is set to
``True`` will not be tracked. For example, it is not possible to debug
coroutines defined in the :mod:`asyncio` module, because the module must be
imported before the flag value can be changed.
Example with the bug::
import asyncio
asyncio.tasks._DEBUG = True
@asyncio.coroutine
def test():
print("never scheduled")
test()
Output in debug mode::
Coroutine 'test' defined at test.py:4 was never yielded from
The fix is to call the :func:`async` function or create a :class:`Task` object
with this coroutine object.
Detect exceptions not consumed
------------------------------
Python usually calls :func:`sys.displayhook` on unhandled exceptions. If
:meth:`Future.set_exception` is called, but the exception is not consumed,
:func:`sys.displayhook` is not called. Instead, a log is emitted when the
future is deleted by the garbage collector, with the traceback where the
exception was raised. See the :ref:`asyncio logger <asyncio-logger>`.
Example of unhandled exception::
import asyncio
@asyncio.coroutine
def bug():
raise Exception("not consumed")
loop = asyncio.get_event_loop()
asyncio.async(bug())
loop.run_forever()
Output::
Future/Task exception was never retrieved:
Traceback (most recent call last):
File "/usr/lib/python3.4/asyncio/tasks.py", line 279, in _step
result = next(coro)
File "/usr/lib/python3.4/asyncio/tasks.py", line 80, 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 to
coroutine in another coroutine and use classic try/except::
@asyncio.coroutine
def handle_exception():
try:
yield from bug()
except Exception:
print("exception consumed")
loop = asyncio.get_event_loop()
asyncio.async(handle_exception())
loop.run_forever()
Another option is to use the :meth:`BaseEventLoop.run_until_complete`
function::
task = asyncio.async(bug())
try:
loop.run_until_complete(task)
except Exception:
print("exception consumed")
See also the :meth:`Future.exception` method.
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Chain coroutines correctly
--------------------------
When a coroutine function calls other coroutine functions and tasks, they
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should be chained explicitly with ``yield from``. Otherwise, the execution is
not guaranteed to be sequential.
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Example with different bugs using :func:`asyncio.sleep` to simulate slow
operations::
import asyncio
@asyncio.coroutine
def create():
yield from asyncio.sleep(3.0)
print("(1) create file")
@asyncio.coroutine
def write():
yield from asyncio.sleep(1.0)
print("(2) write into file")
@asyncio.coroutine
def close():
print("(3) close file")
@asyncio.coroutine
def test():
asyncio.async(create())
asyncio.async(write())
asyncio.async(close())
yield from asyncio.sleep(2.0)
loop.stop()
loop = asyncio.get_event_loop()
asyncio.async(test())
loop.run_forever()
print("Pending tasks at exit: %s" % asyncio.Task.all_tasks(loop))
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loop.close()
Expected output::
(1) create file
(2) write into file
(3) close file
Pending tasks at exit: set()
Actual output::
(3) close file
(2) write into file
Pending tasks at exit: {Task(<create>)<PENDING>}
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 ``yield from``::
@asyncio.coroutine
def test():
yield from asyncio.async(create())
yield from asyncio.async(write())
yield from asyncio.async(close())
yield from asyncio.sleep(2.0)
loop.stop()
Or without ``asyncio.async()``::
@asyncio.coroutine
def test():
yield from create()
yield from write()
yield from close()
yield from asyncio.sleep(2.0)
loop.stop()