"""Support for tasks, coroutines and the scheduler."""
'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
'wait', 'wait_for', 'as_completed', 'sleep', 'async',
'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe',
import concurrent.futures
from .coroutines import coroutine
class Task(futures.Future):
"""A coroutine wrapped in a Future."""
# An important invariant maintained while a Task not done:
# - Either _fut_waiter is None, and _step() is scheduled;
# - or _fut_waiter is some Future, and _step() is *not* scheduled.
# The only transition from the latter to the former is through
# _wakeup(). When _fut_waiter is not None, one of its callbacks
# Weak set containing all tasks alive.
_all_tasks = weakref.WeakSet()
# Dictionary containing tasks that are currently active in
# all running event loops. {EventLoop: Task}
# If False, don't log a message if the task is destroyed whereas its
# status is still pending
_log_destroy_pending = True
def current_task(cls, 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.
loop = events.get_event_loop()
return cls._current_tasks.get(loop)
def all_tasks(cls, loop=None):
"""Return a set of all tasks for an event loop.
By default all tasks for the current event loop are returned.
loop = events.get_event_loop()
return {t for t in cls._all_tasks if t._loop is loop}
def __init__(self, coro, *, loop=None):
assert coroutines.iscoroutine(coro), repr(coro)
super().__init__(loop=loop)
if self._source_traceback:
del self._source_traceback[-1]
self._must_cancel = False
self._loop.call_soon(self._step)
self.__class__._all_tasks.add(self)
# On Python 3.3 or older, objects with a destructor that are part of a
# reference cycle are never destroyed. That's not the case any more on
# Python 3.4 thanks to the PEP 442.
if self._state == futures._PENDING and self._log_destroy_pending:
'message': 'Task was destroyed but it is pending!',
if self._source_traceback:
context['source_traceback'] = self._source_traceback
self._loop.call_exception_handler(context)
futures.Future.__del__(self)
return base_tasks._task_repr_info(self)
def 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
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.
return base_tasks._task_get_stack(self, limit)
def 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
return base_tasks._task_print_stack(self, limit, file)
"""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()
self._log_traceback = False
if self._fut_waiter is not None:
if self._fut_waiter.cancel():
# Leave self._fut_waiter; it may be a Task that
# catches and ignores the cancellation so we may have
# to cancel it again later.
# It must be the case that self._step is already scheduled.
def _step(self, exc=None):
assert not self.done(), \
'_step(): already done: {!r}, {!r}'.format(self, exc)
if not isinstance(exc, futures.CancelledError):
exc = futures.CancelledError()
self._must_cancel = False
self.__class__._current_tasks[self._loop] = self
# Call either coro.throw(exc) or coro.send(None).
# We use the `send` method directly, because coroutines
# don't have `__iter__` and `__next__` methods.
except StopIteration as exc:
# Task is cancelled right before coro stops.
self._must_cancel = False
self.set_exception(futures.CancelledError())
self.set_result(exc.value)
except futures.CancelledError:
super().cancel() # I.e., Future.cancel(self).
except BaseException as exc:
blocking = getattr(result, '_asyncio_future_blocking', None)
# Yielded Future must come from Future.__iter__().
if result._loop is not self._loop:
'Task {!r} got Future {!r} attached to a '
'different loop'.format(self, result)))
'Task cannot await on itself: {!r}'.format(
result._asyncio_future_blocking = False
result.add_done_callback(self._wakeup)
self._fut_waiter = result
if self._fut_waiter.cancel():
self._must_cancel = False
'yield was used instead of yield from '
'in task {!r} with {!r}'.format(self, result)))
# Bare yield relinquishes control for one event loop iteration.
self._loop.call_soon(self._step)
elif inspect.isgenerator(result):
# Yielding a generator is just wrong.
'yield was used instead of yield from for '
'generator in task {!r} with {!r}'.format(
# Yielding something else is an error.
'Task got bad yield: {!r}'.format(result)))
self.__class__._current_tasks.pop(self._loop)
self = None # Needed to break cycles when an exception occurs.
def _wakeup(self, future):
# This may also be a cancellation.
# Don't pass the value of `future.result()` explicitly,
# as `Future.__iter__` and `Future.__await__` don't need it.
# If we call `_step(value, None)` instead of `_step()`,
# Python eval loop would use `.send(value)` method call,
# instead of `__next__()`, which is slower for futures
# that return non-generator iterators from their `__iter__`.
self = None # Needed to break cycles when an exception occurs.
# _CTask is needed for tests.
Task = _CTask = _asyncio.Task
# wait() and as_completed() similar to those in PEP 3148.
FIRST_COMPLETED = concurrent.futures.FIRST_COMPLETED
FIRST_EXCEPTION = concurrent.futures.FIRST_EXCEPTION
ALL_COMPLETED = concurrent.futures.ALL_COMPLETED
def wait(fs, *, loop=None, timeout=None, return_when=ALL_COMPLETED):
"""Wait for the Futures and coroutines given by fs to complete.
The sequence futures must not be empty.
Coroutines will be wrapped in Tasks.
Returns two sets of Future: (done, pending).
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.
if futures.isfuture(fs) or coroutines.iscoroutine(fs):
raise TypeError("expect a list of futures, not %s" % type(fs).__name__)
raise ValueError('Set of coroutines/Futures is empty.')
if return_when not in (FIRST_COMPLETED, FIRST_EXCEPTION, ALL_COMPLETED):
raise ValueError('Invalid return_when value: {}'.format(return_when))
loop = events.get_event_loop()
fs = {ensure_future(f, loop=loop) for f in set(fs)}
return (yield from _wait(fs, timeout, return_when, loop))
def _release_waiter(waiter, *args):
def 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.
loop = events.get_event_loop()
waiter = loop.create_future()
timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
cb = functools.partial(_release_waiter, waiter)
fut = ensure_future(fut, loop=loop)
fut.add_done_callback(cb)
# wait until the future completes or the timeout
except futures.CancelledError:
fut.remove_done_callback(cb)
fut.remove_done_callback(cb)
raise futures.TimeoutError()
def _wait(fs, timeout, return_when, loop):
"""Internal helper for wait() and wait_for().
The fs argument must be a collection of Futures.
assert fs, 'Set of Futures is empty.'
waiter = loop.create_future()
timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
return_when == FIRST_COMPLETED or
return_when == FIRST_EXCEPTION and (not f.cancelled() and
f.exception() is not None)):
if timeout_handle is not None:
f.add_done_callback(_on_completion)
if timeout_handle is not None:
done, pending = set(), set()
f.remove_done_callback(_on_completion)
# This is *not* a @coroutine! It is just an iterator (yielding Futures).
def 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 = yield from f # The 'yield from' may raise.
If a timeout is specified, the 'yield from' will raise
TimeoutError when the timeout occurs before all Futures are done.
Note: The futures 'f' are not necessarily members of fs.
if futures.isfuture(fs) or coroutines.iscoroutine(fs):
raise TypeError("expect a list of futures, not %s" % type(fs).__name__)
loop = loop if loop is not None else events.get_event_loop()
todo = {ensure_future(f, loop=loop) for f in set(fs)}
from .queues import Queue # Import here to avoid circular import problem.
f.remove_done_callback(_on_completion)
done.put_nowait(None) # Queue a dummy value for _wait_for_one().
todo.clear() # Can't do todo.remove(f) in the loop.
return # _on_timeout() was here first.
if not todo and timeout_handle is not None:
f = yield from done.get()
# Dummy value from _on_timeout().
raise futures.TimeoutError
return f.result() # May raise f.exception().
f.add_done_callback(_on_completion)
if todo and timeout is not None:
timeout_handle = loop.call_later(timeout, _on_timeout)
for _ in range(len(todo)):
def sleep(delay, result=None, *, loop=None):
"""Coroutine that completes after a given time (in seconds)."""
loop = events.get_event_loop()
future = loop.create_future()
h = future._loop.call_later(delay,
futures._set_result_unless_cancelled,
return (yield from future)
def async_(coro_or_future, *, loop=None):
"""Wrap a coroutine in a future.
If the argument is a Future, it is returned directly.
This function is deprecated in 3.5. Use asyncio.ensure_future() instead.
warnings.warn("asyncio.async() function is deprecated, use ensure_future()",
return ensure_future(coro_or_future, loop=loop)
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