18.5.3. Tasks and coroutines
18.5.3.1. Coroutines
Coroutines used with asyncio
may be implemented using the
async def
statement, or by using generators.
The 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 @asyncio.coroutine
, although this is not strictly enforced.
The decorator enables compatibility with 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
async def
or decorated with@asyncio.coroutine
). If disambiguation is needed we will call this a coroutine function (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 (
iscoroutine()
returnsTrue
).
Things a coroutine can do:
result = await future
orresult = 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 aCancelledError
exception.) Note that tasks are futures, and everything said about futures also applies to tasks.result = await coroutine
orresult = yield from coroutine
– wait for another coroutine to produce a result (or raise an exception, which will be propagated). Thecoroutine
expression must be a call to another coroutine.return expression
– produce a result to the coroutine that is waiting for this one usingawait
oryield from
.raise exception
– raise an exception in the coroutine that is waiting for this one usingawait
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 ensure_future()
function or the BaseEventLoop.create_task()
method.
Coroutines (and tasks) can only run when the event loop is running.
-
@
asyncio.
coroutine
Decorator to mark generator-based coroutines. This enables the generator use
yield from
to callasync def
coroutines, and also enables the generator to be called byasync def
coroutines, for instance using anawait
expression.There is no need to decorate
async def
coroutines themselves.If the generator is not yielded from before it is destroyed, an error message is logged. See Detect coroutines never scheduled.
Note
In this documentation, some methods are documented as coroutines,
even if they are plain Python functions returning a 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 ensure_future()
.
18.5.3.1.1. 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()
See also
The Hello World with call_soon()
example uses the BaseEventLoop.call_soon()
method to schedule a
callback.
18.5.3.1.2. Example: Coroutine displaying the current date
Example of coroutine displaying the current date every second during 5 seconds
using the 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()
The same coroutine implemented using a generator:
@asyncio.coroutine
def display_date(loop):
end_time = loop.time() + 5.0
while True:
print(datetime.datetime.now())
if (loop.time() + 1.0) >= end_time:
break
yield from asyncio.sleep(1)
See also
The display the current date with call_later() example uses a callback with the
BaseEventLoop.call_later()
method.
18.5.3.1.3. 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:
![../_images/tulip_coro.png](tulip_coro.png)
The “Task” is created by the BaseEventLoop.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 BaseEventLoop.call_later()
to wake up the task in 1 second.
18.5.3.2. InvalidStateError
-
exception
asyncio.
InvalidStateError
The operation is not allowed in this state.
18.5.3.3. TimeoutError
-
exception
asyncio.
TimeoutError
The operation exceeded the given deadline.
Note
This exception is different from the builtin TimeoutError
exception!
18.5.3.4. Future
-
class
asyncio.
Future
(*, loop=None) This class is almost compatible with
concurrent.futures.Future
.Differences:
result()
andexception()
do not take a timeout argument and raise an exception when the future isn’t done yet.- Callbacks registered with
add_done_callback()
are always called via the event loop’scall_soon_threadsafe()
. - This class is not compatible with the
wait()
andas_completed()
functions in theconcurrent.futures
package.
This class is not thread safe.
-
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 returnTrue
.
-
cancelled
() Return
True
if the future was cancelled.
-
done
() Return True if the future is done.
Done means either that a result / exception are available, or that the future was cancelled.
-
result
() Return the result this future represents.
If the future has been cancelled, raises
CancelledError
. If the future’s result isn’t yet available, raisesInvalidStateError
. If the future is done and has an exception set, this exception is raised.
-
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, raisesCancelledError
. If the future isn’t done yet, raisesInvalidStateError
.
-
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
call_soon()
.Use functools.partial to pass parameters to the callback. For example,
fut.add_done_callback(functools.partial(print, "Future:", flush=True))
will callprint("Future:", fut, flush=True)
.
-
remove_done_callback
(fn) Remove all instances of a callback from the “call when done” list.
Returns the number of callbacks removed.
-
set_result
(result) Mark the future done and set its result.
If the future is already done when this method is called, raises
InvalidStateError
.
-
set_exception
(exception) Mark the future done and set an exception.
If the future is already done when this method is called, raises
InvalidStateError
.
18.5.3.4.1. Example: Future with run_until_complete()
Example combining a Future
and a coroutine function:
import asyncio
@asyncio.coroutine
def slow_operation(future):
yield from 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
run_until_complete()
method waits for the completion of
the future.
Note
The run_until_complete()
method uses internally the
add_done_callback()
method to be notified when the future is
done.
18.5.3.4.2. Example: Future with run_forever()
The previous example can be written differently using the
Future.add_done_callback()
method to describe explicitly the control
flow:
import asyncio
@asyncio.coroutine
def slow_operation(future):
yield from 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.
18.5.3.5. Task
-
class
asyncio.
Task
(coro, *, loop=None) Schedule the execution of a coroutine: wrap it in a future. A task is a subclass of
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 completition 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
cancel()
will throw aCancelledError
to the wrapped coroutine.cancelled()
only returnsTrue
if the wrapped coroutine did not catch theCancelledError
exception, or raised aCancelledError
exception.If a pending task is destroyed, the execution of its wrapped coroutine did not complete. It is probably a bug and a warning is logged: see Pending task destroyed.
Don’t directly create
Task
instances: use theensure_future()
function or theBaseEventLoop.create_task()
method.This class is not thread safe.
-
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 aTask
.
-
cancel
() Request that this task cancel itself.
This arranges for a
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
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,
cancelled()
will not returnTrue
(unless the task was already cancelled). A task will be marked as cancelled when the wrapped coroutine terminates with aCancelledError
exception (even ifcancel()
was not called).
-
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.
-
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.
-
classmethod
18.5.3.5.1. Example: Parallel execution of tasks
Example executing 3 tasks (A, B, C) in parallel:
import asyncio
@asyncio.coroutine
def factorial(name, number):
f = 1
for i in range(2, number+1):
print("Task %s: Compute factorial(%s)..." % (name, i))
yield from asyncio.sleep(1)
f *= i
print("Task %s: factorial(%s) = %s" % (name, number, f))
loop = asyncio.get_event_loop()
tasks = [
asyncio.ensure_future(factorial("A", 2)),
asyncio.ensure_future(factorial("B", 3)),
asyncio.ensure_future(factorial("C", 4))]
loop.run_until_complete(asyncio.wait(tasks))
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.
18.5.3.6. Task functions
Note
In the functions below, the optional loop argument allows to explicitly set the event loop object used by the underlying task or coroutine. If it’s not provided, the default event loop is used.
-
asyncio.
as_completed
(fs, *, loop=None, timeout=None) Return an iterator whose values, when waited for, are
Future
instances.Raises
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.
-
asyncio.
ensure_future
(coro_or_future, *, loop=None) Schedule the execution of a coroutine object: wrap it in a future. Return a
Task
object.If the argument is a
Future
, it is returned directly.New in version 3.4.4.
See also
The
BaseEventLoop.create_task()
method.
-
asyncio.
async
(coro_or_future, *, loop=None) A deprecated alias to
ensure_future()
.Deprecated since version 3.4.4.
-
asyncio.
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
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.)
-
asyncio.
iscoroutine
(obj) Return
True
if obj is a coroutine object, which may be based on a generator or anasync def
coroutine.
-
asyncio.
iscoroutinefunction
(func) Return
True
if func is determined to be a coroutine function, which may be a decorated generator function or anasync def
function.
-
coroutine
asyncio.
sleep
(delay, result=None, *, loop=None) Create a 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 granularity of the event loop.
This function is a coroutine.
-
asyncio.
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 ofsomething()
, the cancellation did not happen. But its caller is still cancelled, so the yield-from expression still raisesCancelledError
. Note: Ifsomething()
is cancelled by other means this will still cancelshield()
.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
-
coroutine
asyncio.
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
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
concurrent.futures
module:Constant Description FIRST_COMPLETED
The function will return when any future finishes or is cancelled. 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 ALL_COMPLETED
.ALL_COMPLETED
The function will return when all futures finish or are cancelled. This function is a coroutine.
Usage:
done, pending = yield from asyncio.wait(fs)
Note
This does not raise
asyncio.TimeoutError
! Futures that aren’t done when the timeout occurs are returned in the second set.
-
coroutine
asyncio.
wait_for
(fut, timeout, *, loop=None) Wait for the single
Future
or coroutine object to complete with timeout. If timeout isNone
, block until the future completes.Coroutine will be wrapped in
Task
.Returns result of the Future or coroutine. When a timeout occurs, it cancels the task and raises
asyncio.TimeoutError
. To avoid the task cancellation, wrap it inshield()
.If the wait is cancelled, the future fut is also cancelled.
This function is a coroutine, usage:
result = yield from asyncio.wait_for(fut, 60.0)
Changed in version 3.4.3: If the wait is cancelled, the future fut is now also cancelled.
-
asyncio.
run_coroutine_threadsafe
(coro, loop) Submit a coroutine object to a given event loop.
Return a
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 concurrency and multithreading section of the documentation.
Note
Unlike the functions above,
run_coroutine_threadsafe()
requires the loop argument to be passed explicitely.New in version 3.4.4,: 3.5.1