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Support for tasks, coroutines and the scheduler.
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, /)
all_tasks(loop=None) Return a set of all tasks for the loop.
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(coro, *, name=None)
Schedule the execution of a coroutine object in a spawn task.
Return a Task object.
current_task(loop=None) Return a currently executed task.
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(*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(coro, loop)
Submit a coroutine object to a given event loop.
Return a concurrent.futures.Future to access the result.
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(delay, result=None, *, loop=None) Coroutine that completes after a given time (in seconds).
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(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.
ALL_COMPLETED = 'ALL_COMPLETED'
FIRST_COMPLETED = 'FIRST_COMPLETED'
FIRST_EXCEPTION = 'FIRST_EXCEPTION'