
Tasks and coroutines
********************


Coroutines
==========

A coroutine is a generator that follows certain conventions.  For
documentation purposes, all coroutines should be decorated with
"@asyncio.coroutine", but this cannot be strictly enforced.

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
  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 = 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 = 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 "yield from".

* "raise exception" -- raise an exception in the coroutine that is
  waiting for this one using "yield from".

Calling a coroutine does not start its code running -- it is just a
generator, and the coroutine object returned by the call is really a
generator object, which doesn't do anything until you iterate over it.
In the case of a coroutine object, there are two basic ways to start
it running: call "yield from coroutine" from another coroutine
(assuming the other coroutine is already running!), or convert it to a
"Task".

Coroutines (and tasks) can only run when the event loop is running.

@asyncio.coroutine

   Decorator to mark coroutines.

   If the coroutine 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
  "async()".


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())

See also: *Hello World example using a 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))
   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 "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.


InvalidStateError
=================

exception exception asyncio.InvalidStateError

   The operation is not allowed in this state.


Future
======

class 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.

   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()".

   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: 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.Task(slow_operation(future))
   loop.run_until_complete(future)
   print(future.result())
   loop.close()

The coroutine function is responsible of 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.


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.Task(slow_operation(future))
   future.add_done_callback(got_result)
   try:
       loop.run_forever()
   finally:
       loop.close()

In this example, the future is responsible to display the result and
to stop the loop.

Note: The "slow_operation" coroutine object 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 object.


Task
====

class class asyncio.Task(coro, *, loop=None)

   A coroutine object wrapped in a "Future". Subclass of "Future".

   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".

   get_stack(self, *, limit=None)

      Return the list of stack frames for this task's coroutine.

      If the coroutine is active, 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 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(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))

   tasks = [
       asyncio.Task(factorial("A", 2)),
       asyncio.Task(factorial("B", 3)),
       asyncio.Task(factorial("C", 4))]

   loop = asyncio.get_event_loop()
   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.


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 "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.async(coro_or_future, *, loop=None)

   Wrap a *coroutine object* in a future.

   If the argument is a "Future", it is returned directly.

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*.

asyncio.iscoroutinefunction(obj)

   Return "True" if *func* is a decorated *coroutine function*.

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*.

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).

   *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 |
   +-------------------------------+------------------------------------------+
   | "FIRST_EXCEPTION"             | The function will return when any future |
   +-------------------------------+------------------------------------------+
   | "ALL_COMPLETED"               | The function will return when all        |
   +-------------------------------+------------------------------------------+

   This function is a *coroutine*.

   Usage:

      done, pending = yield from asyncio.wait(fs)

   Note: This does not raise "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 "TimeoutError". To avoid the task
   cancellation, wrap it in "shield()".

   This function is a *coroutine*.

   Usage:

      result = yield from asyncio.wait_for(fut, 60.0)
