.. currentmodule:: asyncio
Source code: :source:`Lib/asyncio/tasks.py`
Source code: :source:`Lib/asyncio/coroutines.py`
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` returnsTrue). - 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 futureorresult = 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 aCancelledErrorexception.) Note that tasks are futures, and everything said about futures also applies to tasks.result = await coroutineorresult = yield from coroutine-- wait for another coroutine to produce a result (or raise an exception, which will be propagated). Thecoroutineexpression must be a call to another coroutine.return expression-- produce a result to the coroutine that is waiting for this one using :keyword:`await` oryield from.raise exception-- raise an exception in the coroutine that is waiting for this one using :keyword:`await` oryield 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`.
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.
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 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:
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.
.. exception:: InvalidStateError The operation is not allowed in this state.
.. exception:: TimeoutError The operation exceeded the given deadline.
Note
This exception is different from the builtin :exc:`TimeoutError` exception!
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 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.
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.
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.
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|
+-----------------------------+----------------------------------------+
| Constant | Description |
+=============================+========================================+
| :const:`FIRST_COMPLETED` | The function will return when any |
| | future finishes or is cancelled. |
+-----------------------------+----------------------------------------+
| :const:`FIRST_EXCEPTION` | The function will return when any |
| | future finishes by raising an |
| | exception. If no future raises an |
| | exception then it is equivalent to |
| | :const:`ALL_COMPLETED`. |
+-----------------------------+----------------------------------------+
| :const:`ALL_COMPLETED` | The function will return when all |
| | futures finish or are cancelled. |
+-----------------------------+----------------------------------------+
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.