asyncio.tasks

Support for tasks, coroutines and the scheduler.

Classes

Task

A coroutine wrapped in a Future.

add_done_callback(...)

  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.

cancel(self, /, msg=None)

  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, Task.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).

cancelled(self, /)

  Return True if the future was cancelled.

done(self, /)

  Return True if the future is done.

  Done means either that a result / exception are available, or that the
  future was cancelled.

exception(self, /)

  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.

get_coro(self, /)

get_loop(self, /)

  Return the event loop the Future is bound to.

get_name(self, /)

get_stack(self, /, *, 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(self, /, *, 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.

remove_done_callback(self, fn, /)

  Remove all instances of a callback from the "call when done" list.

  Returns the number of callbacks removed.

result(self, /)

  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.

set_exception(self, exception, /)

set_name(self, value, /)

set_result(self, result, /)

Functions

all_tasks

all_tasks(loop=None)

  Return a set of all tasks for the loop.

as_completed

as_completed(fs, *, loop=None, timeout=None)

  Return an iterator whose values are coroutines.

      When waiting for the yielded coroutines you'll get the results (or
      exceptions!) of the original Futures (or coroutines), in the order
      in which and as soon as they complete.

      This differs from PEP 3148; the proper way to use this is:

          for f in as_completed(fs):
              result = await f  # The 'await' may raise.
              # Use result.

      If a timeout is specified, the 'await' will raise
      TimeoutError when the timeout occurs before all Futures are done.

      Note: The futures 'f' are not necessarily members of fs.

create_task

create_task(coro, *, name=None)

  Schedule the execution of a coroutine object in a spawn task.

      Return a Task object.

current_task

current_task(loop=None)

  Return a currently executed task.

ensure_future

ensure_future(coro_or_future, *, loop=None)

  Wrap a coroutine or an awaitable in a future.

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

gather

gather(*coros_or_futures, loop=None, return_exceptions=False)

  Return a future aggregating results from the given coroutines/futures.

      Coroutines will be wrapped in a future and scheduled in the event
      loop. They will not necessarily be scheduled in the same order as
      passed in.

      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.)

      If *return_exceptions* is False, cancelling gather() after it
      has been marked done won't cancel any submitted awaitables.
      For instance, gather can be marked done after propagating an
      exception to the caller, therefore, calling ``gather.cancel()``
      after catching an exception (raised by one of the awaitables) from
      gather won't cancel any other awaitables.

run_coroutine_threadsafe

run_coroutine_threadsafe(coro, loop)

  Submit a coroutine object to a given event loop.

      Return a concurrent.futures.Future to access the result.

shield

shield(arg, *, loop=None)

  Wait for a future, shielding it from cancellation.

      The statement

          res = await shield(something())

      is exactly equivalent to the statement

          res = await something()

      *except* that if the coroutine containing it is cancelled, the
      task running in something() is not cancelled.  From the POV 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 = await shield(something())
          except CancelledError:
              res = None

sleep

sleep(delay, result=None, *, loop=None)

  Coroutine that completes after a given time (in seconds).

wait

wait(fs, *, loop=None, timeout=None, return_when='ALL_COMPLETED')

  Wait for the Futures and coroutines given by fs to complete.

      The fs iterable must not be empty.

      Coroutines will be wrapped in Tasks.

      Returns two sets of Future: (done, pending).

      Usage:

          done, pending = await asyncio.wait(fs)

      Note: This does not raise TimeoutError! Futures that aren't done
      when the timeout occurs are returned in the second set.

wait_for

wait_for(fut, timeout, *, loop=None)

  Wait for the single Future or coroutine to complete, with timeout.

      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().

      If the wait is cancelled, the task is also cancelled.

      This function is a coroutine.

Other members

ALL_COMPLETED = 'ALL_COMPLETED'

FIRST_COMPLETED = 'FIRST_COMPLETED'

FIRST_EXCEPTION = 'FIRST_EXCEPTION'

Modules