mirror of https://github.com/python/cpython
211 lines
5.8 KiB
ReStructuredText
211 lines
5.8 KiB
ReStructuredText
.. currentmodule:: asyncio
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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 explain how to avoid them.
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Handle correctly blocking functions
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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 <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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:func:`BaseEventLoop.run_in_executor` function.
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.. _asyncio-logger:
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Logger
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------
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.. data:: asyncio.logger.log
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:class:`logging.Logger` instance used by :mod:`asyncio` to log messages.
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The logger name is ``'asyncio'``.
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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 but not passed to :func:`async` or to the
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:class:`Task` constructor, it is not scheduled and it is probably a bug.
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To detect such bug, set :data:`asyncio.tasks._DEBUG` to ``True``. When the
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coroutine object is destroyed by the garbage collector, a log will be emitted
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with the traceback where the coroutine function was called. See the
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:ref:`asyncio logger <asyncio-logger>`.
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The debug flag changes the behaviour of the :func:`coroutine` decorator. The
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debug flag value is only when then coroutine function is defined, not when it
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is called. Coroutine functions defined before the debug flag is set to
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``True`` will not be tracked. For example, it is not possible to debug
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coroutines defined in the :mod:`asyncio` module, because the module must be
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imported before the flag value can be changed.
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Example with the bug::
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import asyncio
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asyncio.tasks._DEBUG = True
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@asyncio.coroutine
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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' defined at test.py:4 was never yielded from
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The fix is to call the :func:`async` function or create a :class:`Task` object
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with this coroutine object.
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Detect exceptions not consumed
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------------------------------
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Python usually calls :func:`sys.displayhook` on unhandled exceptions. If
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:meth:`Future.set_exception` is called, but the exception is not consumed,
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:func:`sys.displayhook` is not called. Instead, a log is emitted when the
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future is deleted by the garbage collector, with the traceback where the
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exception was raised. See the :ref:`asyncio logger <asyncio-logger>`.
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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.async(bug())
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loop.run_forever()
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Output::
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Future/Task exception was never retrieved:
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Traceback (most recent call last):
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File "/usr/lib/python3.4/asyncio/tasks.py", line 279, in _step
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result = next(coro)
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File "/usr/lib/python3.4/asyncio/tasks.py", line 80, 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 to
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coroutine in another coroutine and use classic try/except::
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@asyncio.coroutine
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def handle_exception():
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try:
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yield from 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.async(handle_exception())
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loop.run_forever()
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Another option is to use the :meth:`BaseEventLoop.run_until_complete`
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function::
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task = asyncio.async(bug())
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try:
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loop.run_until_complete(task)
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except Exception:
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print("exception consumed")
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See also the :meth:`Future.exception` method.
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Chain correctly coroutines
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--------------------------
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When a coroutine function calls other coroutine functions and tasks, they
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should chained explicitly with ``yield from``. Otherwise, the execution is no
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more guaranteed to be sequential.
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Example with different bugs using sleep to simulate slow operations::
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import asyncio
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@asyncio.coroutine
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def create():
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yield from asyncio.sleep(3.0)
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print("(1) create file")
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@asyncio.coroutine
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def write():
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yield from asyncio.sleep(1.0)
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print("(2) write into file")
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@asyncio.coroutine
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def close():
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print("(3) close file")
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@asyncio.coroutine
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def test():
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asyncio.async(create())
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asyncio.async(write())
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asyncio.async(close())
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yield from asyncio.sleep(2.0)
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loop.stop()
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loop = asyncio.get_event_loop()
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asyncio.async(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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Expected output::
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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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(3) close file
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(2) write into file
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Pending tasks at exit: {Task(<create>)<PENDING>}
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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 ``yield from``::
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@asyncio.coroutine
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def test():
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yield from asyncio.async(create())
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yield from asyncio.async(write())
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yield from asyncio.async(close())
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yield from asyncio.sleep(2.0)
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loop.stop()
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Or without ``asyncio.async()``::
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@asyncio.coroutine
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def test():
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yield from create()
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yield from write()
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yield from close()
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yield from asyncio.sleep(2.0)
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loop.stop()
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.. XXX: Document "poll xxx" log message?
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