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()" 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* ("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()".


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
  "AbstractEventLoop.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()

See also: The display the current date with call_later() example
  uses a callback with the "AbstractEventLoop.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:

[image]

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.


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()" and "exception()" 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’s "call_soon_threadsafe()".

   * This class is not compatible with the "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".


18.5.3.4.1. Example: Future with run_until_complete()
-----------------------------------------------------

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.


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

   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.


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
   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.

   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 "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.


18.5.3.5.1. 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.


18.5.3.6. 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.

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.wrap_future(future, *, loop=None)

   Wrap a "concurrent.futures.Future" object in a "Future" object.

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.

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
   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

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* 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.
