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assyncio doc: rewrite, improve and move coroutine, Future and Task examples

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Doc/library/asyncio-task.rst
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......@@ -56,6 +56,55 @@ it running: call ``yield from coroutine`` from another coroutine
Coroutines (and tasks) can only run when the event loop is running.
.. _asyncio-hello-world-coroutine:
Example: "Hello World" coroutine
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Print ``"Hello World"`` every two seconds using a coroutine::
import asyncio
@asyncio.coroutine
def greet_every_two_seconds():
while True:
print('Hello World')
yield from asyncio.sleep(2)
loop = asyncio.get_event_loop()
loop.run_until_complete(greet_every_two_seconds())
.. seealso::
:ref:`Hello World example using a callback <asyncio-hello-world-callback>`.
Example: Chain coroutines
^^^^^^^^^^^^^^^^^^^^^^^^^
Example chaining coroutines::
import asyncio
@asyncio.coroutine
def compute(x, y):
print("Compute %s + %s ..." % (x, y))
yield from asyncio.sleep(1.0)
return x + y
@asyncio.coroutine
def print_sum(x, y):
result = yield from compute(x, y)
print("%s + %s = %s" % (x, y, result))
loop = asyncio.get_event_loop()
loop.run_until_complete(print_sum(1, 2))
``compute()`` is chained to ``print_sum()``: ``print_sum()`` coroutine waits
until ``compute()`` is completed before returing its result.
InvalidStateError
-----------------
......@@ -148,6 +197,69 @@ Future
:exc:`InvalidStateError`.
Example: Future with run_until_complete()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Example combining a :class:`Future` and a :ref:`coroutine <coroutine>`::
import asyncio
@asyncio.coroutine
def slow_operation(future):
yield from asyncio.sleep(1)
future.set_result('Future in done!')
loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.Task(slow_operation(future))
loop.run_until_complete(future)
print(future.result())
The coroutine is responsible of the computation (which takes 1 second) and
it stores the result into the future. The
:meth:`~BaseEventLoop.run_until_complete` method waits for the completion of
the future.
.. note::
The :meth:`~BaseEventLoop.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
@asyncio.coroutine
def slow_operation(future):
yield from asyncio.sleep(1)
future.set_result('Future in done!')
def got_result(future):
print(future.result())
loop.stop()
loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.Task(slow_operation(future))
future.add_done_callback(got_result)
loop.run_forever()
In this example, the future is responsible to display the result and to stop
the loop.
.. note::
The coroutine is only executed when the event loop starts running, so it is
possible to add a "done callback" to the future after creating the task
scheduling the coroutine.
Task
----
......@@ -195,6 +307,46 @@ Task
goes; by default it goes to sys.stderr.
Example: Parallel execution of tasks
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Example executing 3 tasks (A, B, C) in parallel::
import asyncio
@asyncio.coroutine
def factorial(task, n):
f = 1
for i in range(2, n+1):
print("[%s] Compute factorial(%s)..." % (task, i))
yield from asyncio.sleep(1)
f *= n
print("[%s] factorial(%s) = %s" % (task, n, f))
task_a = asyncio.Task(factorial("A", 2))
task_b = asyncio.Task(factorial("B", 3))
task_c = asyncio.Task(factorial("C", 4))
tasks = [task_a, task_b, task_c]
loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait(tasks))
Output::
[A] Compute factorial(2)...
[B] Compute factorial(2)...
[C] Compute factorial(2)...
[A] factorial(2) = 2
[B] Compute factorial(3)...
[C] Compute factorial(3)...
[B] factorial(3) = 9
[C] Compute factorial(4)...
[C] factorial(4) = 64
When a task is created, it is automatically scheduled for execution. The event
loop stops when all tasks are done.
Task functions
--------------
......@@ -322,145 +474,3 @@ Task functions
the timeout occurs are returned in the second set.
Examples
--------
.. _asyncio-hello-world-coroutine:
Example: Hello World (coroutine)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Print ``Hello World`` every two seconds, using a coroutine::
import asyncio
@asyncio.coroutine
def greet_every_two_seconds():
while True:
print('Hello World')
yield from asyncio.sleep(2)
loop = asyncio.get_event_loop()
loop.run_until_complete(greet_every_two_seconds())
.. seealso::
:ref:`Hello World example using a callback <asyncio-hello-world-callback>`.
Example: Chains coroutines and parallel execution
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Example chaining coroutines and executing multiple coroutines in parallel::
import asyncio
@asyncio.coroutine
def compute(x, y):
print("Start computing %s + %s" % (x, y))
yield from asyncio.sleep(3.0)
return x + y
@asyncio.coroutine
def print_sum(x, y):
result = yield from compute(x, y)
print("%s + %s = %s" % (x, y, result))
@asyncio.coroutine
def wait_task(task):
while 1:
done, pending = yield from asyncio.wait([task], timeout=1.0)
if done:
break
print("Compute in progress...")
asyncio.get_event_loop().stop()
print("Schedule tasks")
task = asyncio.async(print_sum(1, 2))
asyncio.async(wait_task(task))
print("Execute tasks")
loop = asyncio.get_event_loop()
loop.run_forever()
loop.close()
Output::
Schedule tasks
Execute tasks
Start computing 1 + 2
Compute in progress...
Compute in progress...
1 + 2 = 3
Details:
* ``compute()`` is chained to ``print_sum()``: ``print_sum()`` coroutine waits
until ``compute()`` is complete. Coroutines are executed in parallel:
``wait_task()`` is executed while ``compute()`` is blocked in
``asyncio.sleep(3.0)``.
* Coroutines are not executed before the loop is running: ``"Execute tasks"``
is written before ``"Start computing 1 + 2"``.
* ``wait_task()`` stops the event loop when ``print_sum()`` is done.
Example: Future with run_until_complete()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Example combining a :class:`Future` and a :ref:`coroutine <coroutine>`::
import asyncio
@asyncio.coroutine
def slow_operation(future):
yield from asyncio.sleep(1)
future.set_result('Future in done!')
loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.Task(slow_operation(future))
loop.run_until_complete(future)
print(future.result())
loop.close()
The example waits for the completion of the future (which takes 1 second). The
coroutine is responsible of the computation. The event loop is notified when
the future is done (see the :meth:`Future.set_result` method).
Example: Future with run_until_complete()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The previous example can be written differently using the
:meth:`Future.add_done_callback` method::
import asyncio
@asyncio.coroutine
def slow_operation(future):
yield from asyncio.sleep(1)
future.set_result('Future in done!')
def exit(future):
print(future.result())
loop.stop()
loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.Task(slow_operation(future))
future.add_done_callback(exit)
loop.run_forever()
loop.close()
The future is now responsible to display the result and stop the loop using the
``exit()`` callback.
.. note::
The coroutine is only executed when the event loop starts running, so it is
possible to add a "done callback" to the future after creating the task
scheduling the coroutine.
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