mirror of https://github.com/python/cpython
245 lines
6.9 KiB
ReStructuredText
245 lines
6.9 KiB
ReStructuredText
.. currentmodule:: asyncio
|
|
|
|
Develop with asyncio
|
|
====================
|
|
|
|
Asynchronous programming is different than classical "sequential" programming.
|
|
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 in 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())
|
|
|
|
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 correctly blocking functions
|
|
-----------------------------------
|
|
|
|
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
|
|
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.
|
|
|
|
|
|
Chain coroutines correctly
|
|
--------------------------
|
|
|
|
When a coroutine function calls other coroutine functions and tasks, they
|
|
should be chained explicitly with ``yield from``. Otherwise, the execution is
|
|
not guaranteed to be sequential.
|
|
|
|
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))
|
|
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()
|
|
|