FM / library

/usr/share/doc/python3-docs/html/_sources/library/

/usr/share/doc/python3-docs/html/_sources/library

Name	Size	Mode	Actions
2to3.rst.txt	15898	0644	edit dl rm
abc.rst.txt	11805	0644	edit dl rm
aifc.rst.txt	7503	0644	edit dl rm
allos.rst.txt	678	0644	edit dl rm
archiving.rst.txt	440	0644	edit dl rm
argparse.rst.txt	77480	0644	edit dl rm
array.rst.txt	10812	0644	edit dl rm
ast.rst.txt	10159	0644	edit dl rm
asynchat.rst.txt	8528	0644	edit dl rm
asyncio-dev.rst.txt	13398	0644	edit dl rm
asyncio-eventloop.rst.txt	33701	0644	edit dl rm
asyncio-eventloops.rst.txt	7660	0644	edit dl rm
asyncio-protocol.rst.txt	25170	0644	edit dl rm
asyncio-queue.rst.txt	4328	0644	edit dl rm
asyncio-stream.rst.txt	14363	0644	edit dl rm
asyncio-subprocess.rst.txt	15544	0644	edit dl rm
asyncio-sync.rst.txt	9078	0644	edit dl rm
asyncio-task.rst.txt	25406	0644	edit dl rm
asyncio.rst.txt	2272	0644	edit dl rm
asyncore.rst.txt	13624	0644	edit dl rm
atexit.rst.txt	3669	0644	edit dl rm
audioop.rst.txt	10712	0644	edit dl rm
base64.rst.txt	10371	0644	edit dl rm
bdb.rst.txt	12985	0644	edit dl rm
binary.rst.txt	654	0644	edit dl rm
binascii.rst.txt	6539	0644	edit dl rm
binhex.rst.txt	1709	0644	edit dl rm
bisect.rst.txt	5398	0644	edit dl rm
builtins.rst.txt	1465	0644	edit dl rm
bz2.rst.txt	8825	0644	edit dl rm
calendar.rst.txt	10988	0644	edit dl rm
cgi.rst.txt	22650	0644	edit dl rm
cgitb.rst.txt	2918	0644	edit dl rm
chunk.rst.txt	5094	0644	edit dl rm
cmath.rst.txt	9326	0644	edit dl rm
cmd.rst.txt	13795	0644	edit dl rm
code.rst.txt	7817	0644	edit dl rm
codecs.rst.txt	75564	0644	edit dl rm
codeop.rst.txt	3021	0644	edit dl rm
collections.abc.rst.txt	12861	0644	edit dl rm
collections.rst.txt	46063	0644	edit dl rm
colorsys.rst.txt	1820	0644	edit dl rm
compileall.rst.txt	8975	0644	edit dl rm
concurrency.rst.txt	673	0644	edit dl rm
concurrent.futures.rst.txt	16910	0644	edit dl rm
concurrent.rst.txt	171	0644	edit dl rm
configparser.rst.txt	48756	0644	edit dl rm
constants.rst.txt	3331	0644	edit dl rm
contextlib.rst.txt	28175	0644	edit dl rm
copy.rst.txt	3378	0644	edit dl rm
copyreg.rst.txt	2182	0644	edit dl rm
crypt.rst.txt	5122	0644	edit dl rm
crypto.rst.txt	411	0644	edit dl rm
csv.rst.txt	19902	0644	edit dl rm
ctypes.rst.txt	88613	0644	edit dl rm
curses.ascii.rst.txt	8920	0644	edit dl rm
curses.panel.rst.txt	2764	0644	edit dl rm
curses.rst.txt	76397	0644	edit dl rm
custominterp.rst.txt	569	0644	edit dl rm
datatypes.rst.txt	751	0644	edit dl rm
datetime.rst.txt	90524	0644	edit dl rm
dbm.rst.txt	14293	0644	edit dl rm
debug.rst.txt	471	0644	edit dl rm
decimal.rst.txt	74869	0644	edit dl rm
development.rst.txt	704	0644	edit dl rm
difflib.rst.txt	29861	0644	edit dl rm
dis.rst.txt	32567	0644	edit dl rm
distribution.rst.txt	452	0644	edit dl rm
distutils.rst.txt	1974	0644	edit dl rm
doctest.rst.txt	71743	0644	edit dl rm
dummy_threading.rst.txt	784	0644	edit dl rm
email.charset.rst.txt	9293	0644	edit dl rm
email.compat32-message.rst.txt	33493	0644	edit dl rm
email.contentmanager.rst.txt	9112	0644	edit dl rm
email.encoders.rst.txt	2715	0644	edit dl rm
email.errors.rst.txt	4792	0644	edit dl rm
email.examples.rst.txt	1915	0644	edit dl rm
email.generator.rst.txt	13637	0644	edit dl rm
email.header.rst.txt	9188	0644	edit dl rm
email.headerregistry.rst.txt	17957	0644	edit dl rm
email.iterators.rst.txt	2795	0644	edit dl rm
email.message.rst.txt	33008	0644	edit dl rm
email.mime.rst.txt	11721	0644	edit dl rm
email.parser.rst.txt	14092	0644	edit dl rm
email.policy.rst.txt	27022	0644	edit dl rm
email.rst.txt	6785	0644	edit dl rm
email.util.rst.txt	9214	0644	edit dl rm
ensurepip.rst.txt	4987	0644	edit dl rm
enum.rst.txt	32751	0644	edit dl rm
errno.rst.txt	6811	0644	edit dl rm
exceptions.rst.txt	25570	0644	edit dl rm
faulthandler.rst.txt	6136	0644	edit dl rm
fcntl.rst.txt	7152	0644	edit dl rm
filecmp.rst.txt	5623	0644	edit dl rm
fileformats.rst.txt	287	0644	edit dl rm
fileinput.rst.txt	8051	0644	edit dl rm
filesys.rst.txt	961	0644	edit dl rm
fnmatch.rst.txt	3103	0644	edit dl rm
formatter.rst.txt	13242	0644	edit dl rm
fpectl.rst.txt	4180	0644	edit dl rm
fractions.rst.txt	6364	0644	edit dl rm
frameworks.rst.txt	391	0644	edit dl rm
ftplib.rst.txt	17552	0644	edit dl rm
functional.rst.txt	365	0644	edit dl rm
functions.rst.txt	71621	0644	edit dl rm
functools.rst.txt	18503	0644	edit dl rm
gc.rst.txt	9694	0644	edit dl rm
getopt.rst.txt	6557	0644	edit dl rm
getpass.rst.txt	1882	0644	edit dl rm
gettext.rst.txt	25588	0644	edit dl rm
glob.rst.txt	3508	0644	edit dl rm
grp.rst.txt	2417	0644	edit dl rm
gzip.rst.txt	8301	0644	edit dl rm
hashlib.rst.txt	26523	0644	edit dl rm
heapq.rst.txt	13402	0644	edit dl rm
hmac.rst.txt	4020	0644	edit dl rm
html.entities.rst.txt	1319	0644	edit dl rm
html.parser.rst.txt	11275	0644	edit dl rm
html.rst.txt	1304	0644	edit dl rm
http.client.rst.txt	18870	0644	edit dl rm
http.cookiejar.rst.txt	27832	0644	edit dl rm
http.cookies.rst.txt	8645	0644	edit dl rm
http.rst.txt	7096	0644	edit dl rm
http.server.rst.txt	17639	0644	edit dl rm
i18n.rst.txt	408	0644	edit dl rm
idle.rst.txt	25834	0644	edit dl rm
imaplib.rst.txt	20367	0644	edit dl rm
imghdr.rst.txt	2938	0644	edit dl rm
imp.rst.txt	15548	0644	edit dl rm
importlib.rst.txt	53563	0644	edit dl rm
index.rst.txt	2258	0644	edit dl rm
inspect.rst.txt	53928	0644	edit dl rm
internet.rst.txt	992	0644	edit dl rm
intro.rst.txt	2645	0644	edit dl rm
io.rst.txt	40328	0644	edit dl rm
ipaddress.rst.txt	32340	0644	edit dl rm
ipc.rst.txt	649	0644	edit dl rm
itertools.rst.txt	38459	0644	edit dl rm
json.rst.txt	27876	0644	edit dl rm
keyword.rst.txt	617	0644	edit dl rm
language.rst.txt	522	0644	edit dl rm
linecache.rst.txt	2408	0644	edit dl rm
locale.rst.txt	25470	0644	edit dl rm
logging.config.rst.txt	33886	0644	edit dl rm
logging.handlers.rst.txt	42497	0644	edit dl rm
logging.rst.txt	60479	0644	edit dl rm
lzma.rst.txt	17361	0644	edit dl rm
macpath.rst.txt	707	0644	edit dl rm
mailbox.rst.txt	63055	0644	edit dl rm
mailcap.rst.txt	3691	0644	edit dl rm
markup.rst.txt	679	0644	edit dl rm
marshal.rst.txt	4855	0644	edit dl rm
math.rst.txt	14375	0644	edit dl rm
mimetypes.rst.txt	9839	0644	edit dl rm
misc.rst.txt	247	0644	edit dl rm
mm.rst.txt	431	0644	edit dl rm
mmap.rst.txt	11232	0644	edit dl rm
modulefinder.rst.txt	3240	0644	edit dl rm
modules.rst.txt	355	0644	edit dl rm
msilib.rst.txt	18612	0644	edit dl rm
msvcrt.rst.txt	4392	0644	edit dl rm
multiprocessing.rst.txt	104494	0644	edit dl rm
netdata.rst.txt	339	0644	edit dl rm
netrc.rst.txt	3007	0644	edit dl rm
nis.rst.txt	1993	0644	edit dl rm
nntplib.rst.txt	21821	0644	edit dl rm
numbers.rst.txt	8089	0644	edit dl rm
numeric.rst.txt	696	0644	edit dl rm
operator.rst.txt	18989	0644	edit dl rm
optparse.rst.txt	76996	0644	edit dl rm
os.path.rst.txt	16419	0644	edit dl rm
os.rst.txt	137534	0644	edit dl rm
ossaudiodev.rst.txt	17845	0644	edit dl rm
othergui.rst.txt	2822	0644	edit dl rm
parser.rst.txt	15148	0644	edit dl rm
pathlib.rst.txt	30045	0644	edit dl rm
pdb.rst.txt	19454	0644	edit dl rm
persistence.rst.txt	591	0644	edit dl rm
pickle.rst.txt	37440	0644	edit dl rm
pickletools.rst.txt	3729	0644	edit dl rm
pipes.rst.txt	2557	0644	edit dl rm
pkgutil.rst.txt	8730	0644	edit dl rm
platform.rst.txt	9624	0644	edit dl rm
plistlib.rst.txt	7398	0644	edit dl rm
poplib.rst.txt	8197	0644	edit dl rm
posix.rst.txt	3694	0644	edit dl rm
pprint.rst.txt	14274	0644	edit dl rm
profile.rst.txt	27969	0644	edit dl rm
pty.rst.txt	3123	0644	edit dl rm
pwd.rst.txt	2739	0644	edit dl rm
pyclbr.rst.txt	3297	0644	edit dl rm
pydoc.rst.txt	4688	0644	edit dl rm
pyexpat.rst.txt	28620	0644	edit dl rm
python.rst.txt	475	0644	edit dl rm
py_compile.rst.txt	3851	0644	edit dl rm
queue.rst.txt	7268	0644	edit dl rm
quopri.rst.txt	2571	0644	edit dl rm
random.rst.txt	18503	0644	edit dl rm
re.rst.txt	64936	0644	edit dl rm
readline.rst.txt	11943	0644	edit dl rm
reprlib.rst.txt	5162	0644	edit dl rm
resource.rst.txt	12291	0644	edit dl rm
rlcompleter.rst.txt	2294	0644	edit dl rm
runpy.rst.txt	8285	0644	edit dl rm
sched.rst.txt	4842	0644	edit dl rm
secrets.rst.txt	5930	0644	edit dl rm
select.rst.txt	28175	0644	edit dl rm
selectors.rst.txt	8925	0644	edit dl rm
shelve.rst.txt	8390	0644	edit dl rm
shlex.rst.txt	16441	0644	edit dl rm
shutil.rst.txt	26014	0644	edit dl rm
signal.rst.txt	17218	0644	edit dl rm
site.rst.txt	9628	0644	edit dl rm
smtpd.rst.txt	10806	0644	edit dl rm
smtplib.rst.txt	23439	0644	edit dl rm
sndhdr.rst.txt	1994	0644	edit dl rm
socket.rst.txt	68202	0644	edit dl rm
socketserver.rst.txt	23712	0644	edit dl rm
spwd.rst.txt	2958	0644	edit dl rm
sqlite3.rst.txt	39215	0644	edit dl rm
ssl.rst.txt	96021	0644	edit dl rm
stat.rst.txt	9805	0644	edit dl rm
statistics.rst.txt	15301	0644	edit dl rm
stdtypes.rst.txt	179240	0644	edit dl rm
string.rst.txt	35374	0644	edit dl rm
stringprep.rst.txt	4279	0644	edit dl rm
struct.rst.txt	19581	0644	edit dl rm
subprocess.rst.txt	45793	0644	edit dl rm
sunau.rst.txt	7390	0644	edit dl rm
superseded.rst.txt	258	0644	edit dl rm
symbol.rst.txt	975	0644	edit dl rm
symtable.rst.txt	4964	0644	edit dl rm
sys.rst.txt	58273	0644	edit dl rm
sysconfig.rst.txt	8749	0644	edit dl rm
syslog.rst.txt	4302	0644	edit dl rm
tabnanny.rst.txt	2007	0644	edit dl rm
tarfile.rst.txt	31749	0644	edit dl rm
telnetlib.rst.txt	7899	0644	edit dl rm
tempfile.rst.txt	13857	0644	edit dl rm
termios.rst.txt	3751	0644	edit dl rm
test.rst.txt	25764	0644	edit dl rm
text.rst.txt	584	0644	edit dl rm
textwrap.rst.txt	10519	0644	edit dl rm
threading.rst.txt	39257	0644	edit dl rm
time.rst.txt	32967	0644	edit dl rm
timeit.rst.txt	13000	0644	edit dl rm
tk.rst.txt	1639	0644	edit dl rm
tkinter.rst.txt	33027	0644	edit dl rm
tkinter.scrolledtext.rst.txt	1255	0644	edit dl rm
tkinter.tix.rst.txt	22653	0644	edit dl rm
tkinter.ttk.rst.txt	58489	0644	edit dl rm
token.rst.txt	2607	0644	edit dl rm
tokenize.rst.txt	10002	0644	edit dl rm
trace.rst.txt	6914	0644	edit dl rm
traceback.rst.txt	17854	0644	edit dl rm
tracemalloc.rst.txt	22089	0644	edit dl rm
tty.rst.txt	1097	0644	edit dl rm
turtle.rst.txt	71221	0644	edit dl rm
types.rst.txt	9999	0644	edit dl rm
typing.rst.txt	34332	0644	edit dl rm
undoc.rst.txt	780	0644	edit dl rm
unicodedata.rst.txt	5762	0644	edit dl rm
unittest.mock-examples.rst.txt	46206	0644	edit dl rm
unittest.mock.rst.txt	85991	0644	edit dl rm
unittest.rst.txt	93304	0644	edit dl rm
unix.rst.txt	446	0644	edit dl rm
urllib.error.rst.txt	2199	0644	edit dl rm
urllib.parse.rst.txt	27332	0644	edit dl rm
urllib.request.rst.txt	59360	0644	edit dl rm
urllib.robotparser.rst.txt	2969	0644	edit dl rm
urllib.rst.txt	466	0644	edit dl rm
uu.rst.txt	2387	0644	edit dl rm
uuid.rst.txt	8760	0644	edit dl rm
venv.rst.txt	20621	0644	edit dl rm
warnings.rst.txt	20152	0644	edit dl rm
wave.rst.txt	6870	0644	edit dl rm
weakref.rst.txt	21328	0644	edit dl rm
webbrowser.rst.txt	9783	0644	edit dl rm
windows.rst.txt	272	0644	edit dl rm
winreg.rst.txt	24073	0644	edit dl rm
winsound.rst.txt	5134	0644	edit dl rm
wsgiref.rst.txt	33047	0644	edit dl rm
xdrlib.rst.txt	8078	0644	edit dl rm
xml.dom.minidom.rst.txt	10198	0644	edit dl rm
xml.dom.pulldom.rst.txt	5186	0644	edit dl rm
xml.dom.rst.txt	39507	0644	edit dl rm
xml.etree.elementtree.rst.txt	43871	0644	edit dl rm
xml.rst.txt	6071	0644	edit dl rm
xml.sax.handler.rst.txt	15399	0644	edit dl rm
xml.sax.reader.rst.txt	12133	0644	edit dl rm
xml.sax.rst.txt	7159	0644	edit dl rm
xml.sax.utils.rst.txt	3901	0644	edit dl rm
xmlrpc.client.rst.txt	23071	0644	edit dl rm
xmlrpc.rst.txt	475	0644	edit dl rm
xmlrpc.server.rst.txt	14990	0644	edit dl rm
zipapp.rst.txt	17392	0644	edit dl rm
zipfile.rst.txt	24005	0644	edit dl rm
zipimport.rst.txt	5805	0644	edit dl rm
zlib.rst.txt	13912	0644	edit dl rm
_dummy_thread.rst.txt	762	0644	edit dl rm
_thread.rst.txt	6886	0644	edit dl rm
__future__.rst.txt	5245	0644	edit dl rm
__main__.rst.txt	904	0644	edit dl rm

Edit: /usr/share/doc/python3-docs/html/_sources/library/asyncio-task.rst.txt (25406B)

.. currentmodule:: asyncio

Tasks and coroutines
====================

**Source code:** :source:`Lib/asyncio/tasks.py`

**Source code:** :source:`Lib/asyncio/coroutines.py`

.. _coroutine:

Coroutines
----------

Coroutines used with :mod:`asyncio` may be implemented using the
:keyword:`async def` statement, or by using :term:`generators <generator>`.
The :keyword:`async def` type of coroutine was added in Python 3.5, and
is recommended if there is no need to support older Python versions.

Generator-based coroutines should be decorated with :func:`@asyncio.coroutine
<asyncio.coroutine>`, although this is not strictly enforced.
The decorator enables compatibility with :keyword:`async def` coroutines,
and also serves as documentation.  Generator-based
coroutines use the ``yield from`` syntax introduced in :pep:`380`,
instead of the original ``yield`` syntax.

The word "coroutine", like the word "generator", is used for two
different (though related) concepts:

- The function that defines a coroutine
  (a function definition using :keyword:`async def` or
  decorated with ``@asyncio.coroutine``).  If disambiguation is needed
  we will call this a *coroutine function* (:func:`iscoroutinefunction`
  returns ``True``).

- The object obtained by calling a coroutine function.  This object
  represents a computation or an I/O operation (usually a combination)
  that will complete eventually.  If disambiguation is needed we will
  call it a *coroutine object* (:func:`iscoroutine` returns ``True``).

Things a coroutine can do:

- ``result = await future`` or ``result = yield from future`` --
  suspends the coroutine until the
  future is done, then returns the future's result, or raises an
  exception, which will be propagated.  (If the future is cancelled,
  it will raise a ``CancelledError`` exception.)  Note that tasks are
  futures, and everything said about futures also applies to tasks.

- ``result = await coroutine`` or ``result = yield from coroutine`` --
  wait for another coroutine to
  produce a result (or raise an exception, which will be propagated).
  The ``coroutine`` expression must be a *call* to another coroutine.

- ``return expression`` -- produce a result to the coroutine that is
  waiting for this one using :keyword:`await` or ``yield from``.

- ``raise exception`` -- raise an exception in the coroutine that is
  waiting for this one using :keyword:`await` or ``yield from``.

Calling a coroutine does not start its code running --
the coroutine object returned by the call doesn't do anything until you
schedule its execution.  There are two basic ways to start it running:
call ``await coroutine`` or ``yield from coroutine`` from another coroutine
(assuming the other coroutine is already running!), or schedule its execution
using the :func:`ensure_future` function or the :meth:`AbstractEventLoop.create_task`
method.

Coroutines (and tasks) can only run when the event loop is running.

.. decorator:: coroutine

Decorator to mark generator-based coroutines.  This enables
    the generator use :keyword:`!yield from` to call :keyword:`async
    def` coroutines, and also enables the generator to be called by
    :keyword:`async def` coroutines, for instance using an
    :keyword:`await` expression.

There is no need to decorate :keyword:`async def` coroutines themselves.

If the generator is not yielded from before it is destroyed, an error
    message is logged. See :ref:`Detect coroutines never scheduled
    <asyncio-coroutine-not-scheduled>`.

.. note::

In this documentation, some methods are documented as coroutines,
    even if they are plain Python functions returning a :class:`Future`.
    This is intentional to have a freedom of tweaking the implementation
    of these functions in the future. If such a function is needed to be
    used in a callback-style code, wrap its result with :func:`ensure_future`.

.. _asyncio-hello-world-coroutine:

Example: Hello World coroutine
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example of coroutine displaying ``"Hello World"``::

import asyncio

async def hello_world():
        print("Hello World!")

loop = asyncio.get_event_loop()
    # Blocking call which returns when the hello_world() coroutine is done
    loop.run_until_complete(hello_world())
    loop.close()

.. seealso::

The :ref:`Hello World with call_soon() <asyncio-hello-world-callback>`
   example uses the :meth:`AbstractEventLoop.call_soon` method to schedule a
   callback.

.. _asyncio-date-coroutine:

Example: Coroutine displaying the current date
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example of coroutine displaying the current date every second during 5 seconds
using the :meth:`sleep` function::

import asyncio
    import datetime

async def display_date(loop):
        end_time = loop.time() + 5.0
        while True:
            print(datetime.datetime.now())
            if (loop.time() + 1.0) >= end_time:
                break
            await asyncio.sleep(1)

loop = asyncio.get_event_loop()
    # Blocking call which returns when the display_date() coroutine is done
    loop.run_until_complete(display_date(loop))
    loop.close()

.. seealso::

The :ref:`display the current date with call_later()
   <asyncio-date-callback>` example uses a callback with the
   :meth:`AbstractEventLoop.call_later` method.

Example: Chain coroutines
^^^^^^^^^^^^^^^^^^^^^^^^^

Example chaining coroutines::

import asyncio

async def compute(x, y):
        print("Compute %s + %s ..." % (x, y))
        await asyncio.sleep(1.0)
        return x + y

async def print_sum(x, y):
        result = await compute(x, y)
        print("%s + %s = %s" % (x, y, result))

loop = asyncio.get_event_loop()
    loop.run_until_complete(print_sum(1, 2))
    loop.close()

``compute()`` is chained to ``print_sum()``: ``print_sum()`` coroutine waits
until ``compute()`` is completed before returning its result.

Sequence diagram of the example:

.. image:: tulip_coro.png
   :align: center

The "Task" is created by the :meth:`AbstractEventLoop.run_until_complete` method
when it gets a coroutine object instead of a task.

The diagram shows the control flow, it does not describe exactly how things
work internally. For example, the sleep coroutine creates an internal future
which uses :meth:`AbstractEventLoop.call_later` to wake up the task in 1 second.

InvalidStateError
-----------------

.. exception:: InvalidStateError

The operation is not allowed in this state.

TimeoutError
------------

.. exception:: TimeoutError

The operation exceeded the given deadline.

.. note::

This exception is different from the builtin :exc:`TimeoutError` exception!

Future
------

.. class:: Future(\*, loop=None)

This class is *almost* compatible with :class:`concurrent.futures.Future`.

Differences:

- :meth:`result` and :meth:`exception` do not take a timeout argument and
     raise an exception when the future isn't done yet.

- Callbacks registered with :meth:`add_done_callback` are always called
     via the event loop's :meth:`~AbstractEventLoop.call_soon`.

- This class is not compatible with the :func:`~concurrent.futures.wait` and
     :func:`~concurrent.futures.as_completed` functions in the
     :mod:`concurrent.futures` package.

This class is :ref:`not thread safe <asyncio-multithreading>`.

.. method:: cancel()

Cancel the future and schedule callbacks.

If the future is already done or cancelled, return ``False``. Otherwise,
      change the future's state to cancelled, schedule the callbacks and return
      ``True``.

.. method:: cancelled()

Return ``True`` if the future was cancelled.

.. method:: done()

Return ``True`` if the future is done.

Done means either that a result / exception are available, or that the
      future was cancelled.

.. method:: result()

Return the result this future represents.

If the future has been cancelled, raises :exc:`CancelledError`. If the
      future's result isn't yet available, raises :exc:`InvalidStateError`. If
      the future is done and has an exception set, this exception is raised.

.. method:: exception()

Return the exception that was set on this future.

The exception (or ``None`` if no exception was set) is returned only if
      the future is done. If the future has been cancelled, raises
      :exc:`CancelledError`. If the future isn't done yet, raises
      :exc:`InvalidStateError`.

.. method:: add_done_callback(fn)

Add a callback to be run when the future becomes done.

The callback is called with a single argument - the future object. If the
      future is already done when this is called, the callback is scheduled
      with :meth:`~AbstractEventLoop.call_soon`.

:ref:`Use functools.partial to pass parameters to the callback
      <asyncio-pass-keywords>`. For example,
      ``fut.add_done_callback(functools.partial(print, "Future:",
      flush=True))`` will call ``print("Future:", fut, flush=True)``.

.. method:: remove_done_callback(fn)

Remove all instances of a callback from the "call when done" list.

Returns the number of callbacks removed.

.. method:: set_result(result)

Mark the future done and set its result.

If the future is already done when this method is called, raises
      :exc:`InvalidStateError`.

.. method:: set_exception(exception)

Mark the future done and set an exception.

If the future is already done when this method is called, raises
      :exc:`InvalidStateError`.

Example: Future with run_until_complete()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example combining a :class:`Future` and a :ref:`coroutine function
<coroutine>`::

import asyncio

async def slow_operation(future):
        await asyncio.sleep(1)
        future.set_result('Future is done!')

loop = asyncio.get_event_loop()
    future = asyncio.Future()
    asyncio.ensure_future(slow_operation(future))
    loop.run_until_complete(future)
    print(future.result())
    loop.close()

The coroutine function is responsible for the computation (which takes 1 second)
and it stores the result into the future. The
:meth:`~AbstractEventLoop.run_until_complete` method waits for the completion of
the future.

.. note::
   The :meth:`~AbstractEventLoop.run_until_complete` method uses internally the
   :meth:`~Future.add_done_callback` method to be notified when the future is
   done.

Example: Future with run_forever()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The previous example can be written differently using the
:meth:`Future.add_done_callback` method to describe explicitly the control
flow::

import asyncio

async def slow_operation(future):
        await asyncio.sleep(1)
        future.set_result('Future is done!')

def got_result(future):
        print(future.result())
        loop.stop()

loop = asyncio.get_event_loop()
    future = asyncio.Future()
    asyncio.ensure_future(slow_operation(future))
    future.add_done_callback(got_result)
    try:
        loop.run_forever()
    finally:
        loop.close()

In this example, the future is used to link ``slow_operation()`` to
``got_result()``: when ``slow_operation()`` is done, ``got_result()`` is called
with the result.

Task
----

.. class:: Task(coro, \*, loop=None)

Schedule the execution of a :ref:`coroutine <coroutine>`: wrap it in a
   future. A task is a subclass of :class:`Future`.

A task is responsible for executing a coroutine object in an event loop.  If
   the wrapped coroutine yields from a future, the task suspends the execution
   of the wrapped coroutine and waits for the completion of the future. When
   the future is done, the execution of the wrapped coroutine restarts with the
   result or the exception of the future.

Event loops use cooperative scheduling: an event loop only runs one task at
   a time. Other tasks may run in parallel if other event loops are
   running in different threads. While a task waits for the completion of a
   future, the event loop executes a new task.

The cancellation of a task is different from the cancelation of a
   future. Calling :meth:`cancel` will throw a
   :exc:`~concurrent.futures.CancelledError` to the wrapped
   coroutine. :meth:`~Future.cancelled` only returns ``True`` if the
   wrapped coroutine did not catch the
   :exc:`~concurrent.futures.CancelledError` exception, or raised a
   :exc:`~concurrent.futures.CancelledError` exception.

If a pending task is destroyed, the execution of its wrapped :ref:`coroutine
   <coroutine>` did not complete. It is probably a bug and a warning is
   logged: see :ref:`Pending task destroyed <asyncio-pending-task-destroyed>`.

Don't directly create :class:`Task` instances: use the :func:`ensure_future`
   function or the :meth:`AbstractEventLoop.create_task` method.

This class is :ref:`not thread safe <asyncio-multithreading>`.

.. classmethod:: all_tasks(loop=None)

Return a set of all tasks for an event loop.

By default all tasks for the current event loop are returned.

.. classmethod:: current_task(loop=None)

Return the currently running task in an event loop or ``None``.

By default the current task for the current event loop is returned.

``None`` is returned when called not in the context of a :class:`Task`.

.. method:: cancel()

Request that this task cancel itself.

This arranges for a :exc:`~concurrent.futures.CancelledError` to be
      thrown into the wrapped coroutine on the next cycle through the event
      loop. The coroutine then has a chance to clean up or even deny the
      request using try/except/finally.

Unlike :meth:`Future.cancel`, this does not guarantee that the task
      will be cancelled: the exception might be caught and acted upon, delaying
      cancellation of the task or preventing cancellation completely. The task
      may also return a value or raise a different exception.

Immediately after this method is called, :meth:`~Future.cancelled` will
      not return ``True`` (unless the task was already cancelled). A task will
      be marked as cancelled when the wrapped coroutine terminates with a
      :exc:`~concurrent.futures.CancelledError` exception (even if
      :meth:`cancel` was not called).

.. method:: get_stack(\*, limit=None)

Return the list of stack frames for this task's coroutine.

If the coroutine is not done, this returns the stack where it is
      suspended.  If the coroutine has completed successfully or was
      cancelled, this returns an empty list.  If the coroutine was
      terminated by an exception, this returns the list of traceback
      frames.

The frames are always ordered from oldest to newest.

The optional limit gives the maximum number of frames to return; by
      default all available frames are returned.  Its meaning differs depending
      on whether a stack or a traceback is returned: the newest frames of a
      stack are returned, but the oldest frames of a traceback are returned.
      (This matches the behavior of the traceback module.)

For reasons beyond our control, only one stack frame is returned for a
      suspended coroutine.

.. method:: print_stack(\*, limit=None, file=None)

Print the stack or traceback for this task's coroutine.

This produces output similar to that of the traceback module, for the
      frames retrieved by get_stack().  The limit argument is passed to
      get_stack().  The file argument is an I/O stream to which the output
      is written; by default output is written to sys.stderr.

Example: Parallel execution of tasks
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example executing 3 tasks (A, B, C) in parallel::

import asyncio

async def factorial(name, number):
        f = 1
        for i in range(2, number+1):
            print("Task %s: Compute factorial(%s)..." % (name, i))
            await asyncio.sleep(1)
            f *= i
        print("Task %s: factorial(%s) = %s" % (name, number, f))

loop = asyncio.get_event_loop()
    loop.run_until_complete(asyncio.gather(
        factorial("A", 2),
        factorial("B", 3),
        factorial("C", 4),
    ))
    loop.close()

Output::

Task A: Compute factorial(2)...
    Task B: Compute factorial(2)...
    Task C: Compute factorial(2)...
    Task A: factorial(2) = 2
    Task B: Compute factorial(3)...
    Task C: Compute factorial(3)...
    Task B: factorial(3) = 6
    Task C: Compute factorial(4)...
    Task C: factorial(4) = 24

A task is automatically scheduled for execution when it is created. The event
loop stops when all tasks are done.

Task functions
--------------

.. note::

In the functions below, the optional *loop* argument allows explicitly setting
   the event loop object used by the underlying task or coroutine.  If it's
   not provided, the default event loop is used.

.. function:: as_completed(fs, \*, loop=None, timeout=None)

Return an iterator whose values, when waited for, are :class:`Future`
   instances.

Raises :exc:`asyncio.TimeoutError` if the timeout occurs before all Futures
   are done.

Example::

for f in as_completed(fs):
           result = yield from f  # The 'yield from' may raise
           # Use result

.. note::

The futures ``f`` are not necessarily members of fs.

.. function:: ensure_future(coro_or_future, \*, loop=None)

Schedule the execution of a :ref:`coroutine object <coroutine>`: wrap it in
   a future. Return a :class:`Task` object.

If the argument is a :class:`Future`, it is returned directly.

.. versionadded:: 3.4.4

.. versionchanged:: 3.5.1
      The function accepts any :term:`awaitable` object.

.. seealso::

The :meth:`AbstractEventLoop.create_task` method.

.. function:: async(coro_or_future, \*, loop=None)

A deprecated alias to :func:`ensure_future`.

.. deprecated:: 3.4.4

.. function:: wrap_future(future, \*, loop=None)

Wrap a :class:`concurrent.futures.Future` object in a :class:`Future`
   object.

.. function:: gather(\*coros_or_futures, loop=None, return_exceptions=False)

Return a future aggregating results from the given coroutine objects or
   futures.

All futures must share the same event loop.  If all the tasks are done
   successfully, the returned future's result is the list of results (in the
   order of the original sequence, not necessarily the order of results
   arrival).  If *return_exceptions* is true, exceptions in the tasks are
   treated the same as successful results, and gathered in the result list;
   otherwise, the first raised exception will be immediately propagated to the
   returned future.

Cancellation: if the outer Future is cancelled, all children (that have not
   completed yet) are also cancelled.  If any child is cancelled, this is
   treated as if it raised :exc:`~concurrent.futures.CancelledError` -- the
   outer Future is *not* cancelled in this case.  (This is to prevent the
   cancellation of one child to cause other children to be cancelled.)

.. versionchanged:: 3.6.6
      If the *gather* itself is cancelled, the cancellation is propagated
      regardless of *return_exceptions*.

.. function:: iscoroutine(obj)

Return ``True`` if *obj* is a :ref:`coroutine object <coroutine>`,
   which may be based on a generator or an :keyword:`async def` coroutine.

.. function:: iscoroutinefunction(func)

Return ``True`` if *func* is determined to be a :ref:`coroutine function
   <coroutine>`, which may be a decorated generator function or an
   :keyword:`async def` function.

.. function:: run_coroutine_threadsafe(coro, loop)

Submit a :ref:`coroutine object <coroutine>` to a given event loop.

Return a :class:`concurrent.futures.Future` to access the result.

This function is meant to be called from a different thread than the one
   where the event loop is running. Usage::

# Create a coroutine
     coro = asyncio.sleep(1, result=3)
     # Submit the coroutine to a given loop
     future = asyncio.run_coroutine_threadsafe(coro, loop)
     # Wait for the result with an optional timeout argument
     assert future.result(timeout) == 3

If an exception is raised in the coroutine, the returned future will be
   notified. It can also be used to cancel the task in the event loop::

try:
         result = future.result(timeout)
     except asyncio.TimeoutError:
         print('The coroutine took too long, cancelling the task...')
         future.cancel()
     except Exception as exc:
         print('The coroutine raised an exception: {!r}'.format(exc))
     else:
         print('The coroutine returned: {!r}'.format(result))

See the :ref:`concurrency and multithreading <asyncio-multithreading>`
   section of the documentation.

.. note::

Unlike other functions from the module,
      :func:`run_coroutine_threadsafe` requires the *loop* argument to
      be passed explicitly.

.. versionadded:: 3.5.1

.. coroutinefunction:: sleep(delay, result=None, \*, loop=None)

Create a :ref:`coroutine <coroutine>` that completes after a given
   time (in seconds).  If *result* is provided, it is produced to the caller
   when the coroutine completes.

The resolution of the sleep depends on the :ref:`granularity of the event
   loop <asyncio-delayed-calls>`.

This function is a :ref:`coroutine <coroutine>`.

.. coroutinefunction:: shield(arg, \*, loop=None)

Wait for a future, shielding it from cancellation.

The statement::

res = yield from shield(something())

is exactly equivalent to the statement::

res = yield from something()

*except* that if the coroutine containing it is cancelled, the task running
   in ``something()`` is not cancelled.  From the point of view of
   ``something()``, the cancellation did not happen.  But its caller is still
   cancelled, so the yield-from expression still raises
   :exc:`~concurrent.futures.CancelledError`.  Note: If ``something()`` is
   cancelled by other means this will still cancel ``shield()``.

If you want to completely ignore cancellation (not recommended) you can
   combine ``shield()`` with a try/except clause, as follows::

try:
           res = yield from shield(something())
       except CancelledError:
           res = None

.. coroutinefunction:: wait(futures, \*, loop=None, timeout=None,\
                            return_when=ALL_COMPLETED)

Wait for the Futures and coroutine objects given by the sequence *futures*
   to complete.  Coroutines will be wrapped in Tasks. Returns two sets of
   :class:`Future`: (done, pending).

The sequence *futures* must not be empty.

*timeout* can be used to control the maximum number of seconds to wait before
   returning.  *timeout* can be an int or float.  If *timeout* is not specified
   or ``None``, there is no limit to the wait time.

*return_when* indicates when this function should return.  It must be one of
   the following constants of the :mod:`concurrent.futures` module:

.. tabularcolumns:: |l|L|

This function is a :ref:`coroutine <coroutine>`.

Usage::

done, pending = yield from asyncio.wait(fs)

.. note::

This does not raise :exc:`asyncio.TimeoutError`! Futures that aren't done
      when the timeout occurs are returned in the second set.

.. coroutinefunction:: wait_for(fut, timeout, \*, loop=None)

Wait for the single :class:`Future` or :ref:`coroutine object <coroutine>`
   to complete with timeout. If *timeout* is ``None``, block until the future
   completes.

Coroutine will be wrapped in :class:`Task`.

Returns result of the Future or coroutine.  When a timeout occurs, it
   cancels the task and raises :exc:`asyncio.TimeoutError`. To avoid the task
   cancellation, wrap it in :func:`shield`.

If the wait is cancelled, the future *fut* is also cancelled.

This function is a :ref:`coroutine <coroutine>`, usage::

result = yield from asyncio.wait_for(fut, 60.0)

.. versionchanged:: 3.4.3
      If the wait is cancelled, the future *fut* is now also cancelled.

cmd: