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:
async def or
decorated with @asyncio.coroutine). If disambiguation is needed
we will call this a coroutine function (iscoroutinefunction()
returns True).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 await or yield from.raise exception – raise an exception in the coroutine that is
waiting for this one using 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 ensure_future() function or the AbstractEventLoop.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 call async
def coroutines, and also enables the generator to be called by
async def coroutines, for instance using an
await 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().
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 AbstractEventLoop.call_soon() method to schedule a
callback.
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()
See also
The display the current date with call_later() example uses a callback with the
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 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 AbstractEventLoop.call_later() to wake up the task in 1 second.
asyncio.InvalidStateError¶The operation is not allowed in this state.
asyncio.TimeoutError¶The operation exceeded the given deadline.
Note
This exception is different from the builtin TimeoutError exception!
asyncio.Future(*, loop=None)¶This class is almost compatible with concurrent.futures.Future.
Differences:
result() and exception() do not take a timeout argument and
raise an exception when the future isn’t done yet.add_done_callback() are always called
via the event loop’s call_soon_threadsafe().wait() and
as_completed() functions in the
concurrent.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 return
True.
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, raises InvalidStateError. 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, raises
CancelledError. If the future isn’t done yet, raises
InvalidStateError.
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 call print("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.
Example combining a Future and a coroutine function:
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
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.
The previous example can be written differently using the
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.
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 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 cancel() will throw a
CancelledError to the wrapped
coroutine. cancelled() only returns True if the
wrapped coroutine did not catch the
CancelledError exception, or raised a
CancelledError 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 the ensure_future()
function or the AbstractEventLoop.create_task() method.
This class is not thread safe.
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.
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 Task.
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 return True (unless the task was already cancelled). A task will
be marked as cancelled when the wrapped coroutine terminates with a
CancelledError exception (even if
cancel() 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.
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.
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.
Changed in version 3.5.1: The function accepts any awaitable object.
See also
The AbstractEventLoop.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 an async def coroutine.
asyncio.iscoroutinefunction(func)¶Return True if func is determined to be a coroutine function, which may be a decorated generator function or an
async def function.
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 other functions from the module,
run_coroutine_threadsafe() requires the loop argument to
be passed explicitly.
New in version 3.5.1.
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 of
something(), the cancellation did not happen. But its caller is still
cancelled, so the yield-from expression still raises
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
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.
asyncio.wait_for(fut, timeout, *, loop=None)¶Wait for the single Future or coroutine object
to complete with timeout. If timeout is None, 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 in shield().
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.