"""Support for tasks, coroutines and the scheduler."""
__all__ = (
'Task', 'create_task',
'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
'wait', 'wait_for', 'as_completed', 'sleep',
'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe',
'current_task', 'all_tasks',
'create_eager_task_factory', 'eager_task_factory',
'_register_task', '_unregister_task', '_enter_task', '_leave_task',
)
import concurrent.futures
import contextvars
import functools
import inspect
import itertools
import math
import types
import weakref
from types import GenericAlias
from . import base_tasks
from . import coroutines
from . import events
from . import exceptions
from . import futures
from . import queues
from . import timeouts
# Helper to generate new task names
# This uses itertools.count() instead of a "+= 1" operation because the latter
# is not thread safe. See bpo-11866 for a longer explanation.
_task_name_counter = itertools.count(1).__next__
def current_task(loop=None):
"""Return a currently executed task."""
if loop is None:
loop = events.get_running_loop()
return _current_tasks.get(loop)
def all_tasks(loop=None):
"""Return a set of all tasks for the loop."""
if loop is None:
loop = events.get_running_loop()
# capturing the set of eager tasks first, so if an eager task "graduates"
# to a regular task in another thread, we don't risk missing it.
eager_tasks = list(_eager_tasks)
return {t for t in itertools.chain(_scheduled_tasks, eager_tasks)
if futures._get_loop(t) is loop and not t.done()}
class Task(futures._PyFuture): # Inherit Python Task implementation
# from a Python Future implementation.
"""A coroutine wrapped in a Future."""
# An important invariant maintained while a Task not done:
# _fut_waiter is either None or a Future. The Future
# can be either done() or not done().
# The task can be in any of 3 states:
#
# - 1: _fut_waiter is not None and not _fut_waiter.done():
# __step() is *not* scheduled and the Task is waiting for _fut_waiter.
# - 2: (_fut_waiter is None or _fut_waiter.done()) and __step() is scheduled:
# the Task is waiting for __step() to be executed.
# - 3: _fut_waiter is None and __step() is *not* scheduled:
# the Task is currently executing (in __step()).
#
# * In state 1, one of the callbacks of __fut_waiter must be __wakeup().
# * The transition from 1 to 2 happens when _fut_waiter becomes done(),
# as it schedules __wakeup() to be called (which calls __step() so
# we way that __step() is scheduled).
# * It transitions from 2 to 3 when __step() is executed, and it clears
# _fut_waiter to None.
# If False, don't log a message if the task is destroyed while its
# status is still pending
_log_destroy_pending = True
def __init__(self, coro, *, loop=None, name=None, context=None,
eager_start=False):
super().__init__(loop=loop)
if self._source_traceback:
del self._source_traceback[-1]
if not coroutines.iscoroutine(coro):
# raise after Future.__init__(), attrs are required for __del__
# prevent logging for pending task in __del__
self._log_destroy_pending = False
raise TypeError(f"a coroutine was expected, got {coro!r}")
if name is None:
self._name = f'Task-{_task_name_counter()}'
else:
self._name = str(name)
self._num_cancels_requested = 0
self._must_cancel = False
self._fut_waiter = None
self._coro = coro
if context is None:
self._context = contextvars.copy_context()
else:
self._context = context
if eager_start and self._loop.is_running():
self.__eager_start()
else:
self._loop.call_soon(self.__step, context=self._context)
_register_task(self)
def __del__(self):
if self._state == futures._PENDING and self._log_destroy_pending:
context = {
'task': self,
'message': 'Task was destroyed but it is pending!',
}
if self._source_traceback:
context['source_traceback'] = self._source_traceback
self._loop.call_exception_handler(context)
super().__del__()
__class_getitem__ = classmethod(GenericAlias)
def __repr__(self):
return base_tasks._task_repr(self)
def get_coro(self):
return self._coro
def get_context(self):
return self._context
def get_name(self):
return self._name
def set_name(self, value):
self._name = str(value)
def set_result(self, result):
raise RuntimeError('Task does not support set_result operation')
def set_exception(self, exception):
raise RuntimeError('Task does not support set_exception operation')
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
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.
"""
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
to sys.stderr.
"""
return base_tasks._task_print_stack(self, limit, file)
def 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).
This also increases the task's count of cancellation requests.
"""
self._log_traceback = False
if self.done():
return False
self._num_cancels_requested += 1
# These two lines are controversial. See discussion starting at
# https://github.com/python/cpython/pull/31394#issuecomment-1053545331
# Also remember that this is duplicated in _asynciomodule.c.
# if self._num_cancels_requested > 1:
# return False
if self._fut_waiter is not None:
if self._fut_waiter.cancel(msg=msg):
# Leave self._fut_waiter; it may be a Task that
# catches and ignores the cancellation so we may have
# to cancel it again later.
return True
# It must be the case that self.__step is already scheduled.
self._must_cancel = True
self._cancel_message = msg
return True
def cancelling(self):
"""Return the count of the task's cancellation requests.
This count is incremented when .cancel() is called
and may be decremented using .uncancel().
"""
return self._num_cancels_requested
def uncancel(self):
"""Decrement the task's count of cancellation requests.
This should be called by the party that called `cancel()` on the task
beforehand.
Returns the remaining number of cancellation requests.
"""
if self._num_cancels_requested > 0:
self._num_cancels_requested -= 1
if self._num_cancels_requested == 0:
self._must_cancel = False
return self._num_cancels_requested
def __eager_start(self):
prev_task = _swap_current_task(self._loop, self)
try:
_register_eager_task(self)
try:
self._context.run(self.__step_run_and_handle_result, None)
finally:
_unregister_eager_task(self)
finally:
try:
curtask = _swap_current_task(self._loop, prev_task)
assert curtask is self
finally:
if self.done():
self._coro = None
self = None # Needed to break cycles when an exception occurs.
else:
_register_task(self)
def __step(self, exc=None):
if self.done():
raise exceptions.InvalidStateError(
f'__step(): already done: {self!r}, {exc!r}')
if self._must_cancel:
if not isinstance(exc, exceptions.CancelledError):
exc = self._make_cancelled_error()
self._must_cancel = False
self._fut_waiter = None
_enter_task(self._loop, self)
try:
self.__step_run_and_handle_result(exc)
finally:
_leave_task(self._loop, self)
self = None # Needed to break cycles when an exception occurs.
def __step_run_and_handle_result(self, exc):
coro = self._coro
try:
if exc is None:
# We use the `send` method directly, because coroutines
# don't have `__iter__` and `__next__` methods.
result = coro.send(None)
else:
result = coro.throw(exc)
except StopIteration as exc:
if self._must_cancel:
# Task is cancelled right before coro stops.
self._must_cancel = False
super().cancel(msg=self._cancel_message)
else:
super().set_result(exc.value)
except exceptions.CancelledError as exc:
# Save the original exception so we can chain it later.
self._cancelled_exc = exc
super().cancel() # I.e., Future.cancel(self).
except (KeyboardInterrupt, SystemExit) as exc:
super().set_exception(exc)
raise
except BaseException as exc:
super().set_exception(exc)
else:
blocking = getattr(result, '_asyncio_future_blocking', None)
if blocking is not None:
# Yielded Future must come from Future.__iter__().
if futures._get_loop(result) is not self._loop:
new_exc = RuntimeError(
f'Task {self!r} got Future '
f'{result!r} attached to a different loop')
self._loop.call_soon(
self.__step, new_exc, context=self._context)
elif blocking:
if result is self:
new_exc = RuntimeError(
f'Task cannot await on itself: {self!r}')
self._loop.call_soon(
self.__step, new_exc, context=self._context)
else:
result._asyncio_future_blocking = False
result.add_done_callback(
self.__wakeup, context=self._context)
self._fut_waiter = result
if self._must_cancel:
if self._fut_waiter.cancel(
msg=self._cancel_message):
self._must_cancel = False
else:
new_exc = RuntimeError(
f'yield was used instead of yield from '
f'in task {self!r} with {result!r}')
self._loop.call_soon(
self.__step, new_exc, context=self._context)
elif result is None:
# Bare yield relinquishes control for one event loop iteration.
self._loop.call_soon(self.__step, context=self._context)
elif inspect.isgenerator(result):
# Yielding a generator is just wrong.
new_exc = RuntimeError(
f'yield was used instead of yield from for '
f'generator in task {self!r} with {result!r}')
self._loop.call_soon(
self.__step, new_exc, context=self._context)
else:
# Yielding something else is an error.
new_exc = RuntimeError(f'Task got bad yield: {result!r}')
self._loop.call_soon(
self.__step, new_exc, context=self._context)
finally:
self = None # Needed to break cycles when an exception occurs.
def __wakeup(self, future):
try:
future.result()
except BaseException as exc:
# This may also be a cancellation.
self.__step(exc)
else:
# 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.__step()
self = None # Needed to break cycles when an exception occurs.
_PyTask = Task
try:
import _asyncio
except ImportError:
pass
else:
# _CTask is needed for tests.
Task = _CTask = _asyncio.Task
def create_task(coro, *, name=None, context=None):
"""Schedule the execution of a coroutine object in a spawn task.
Return a Task object.
"""
loop = events.get_running_loop()
if context is None:
# Use legacy API if context is not needed
task = loop.create_task(coro, name=name)
else:
task = loop.create_task(coro, name=name, context=context)
return 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
async def wait(fs, *, timeout=None, return_when=ALL_COMPLETED):
"""Wait for the Futures or Tasks given by fs to complete.
The fs iterable must not be empty.
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.
"""
if futures.isfuture(fs) or coroutines.iscoroutine(fs):
raise TypeError(f"expect a list of futures, not {type(fs).__name__}")
if not fs:
raise ValueError('Set of Tasks/Futures is empty.')
if return_when not in (FIRST_COMPLETED, FIRST_EXCEPTION, ALL_COMPLETED):
raise ValueError(f'Invalid return_when value: {return_when}')
fs = set(fs)
if any(coroutines.iscoroutine(f) for f in fs):
raise TypeError("Passing coroutines is forbidden, use tasks explicitly.")
loop = events.get_running_loop()
return await _wait(fs, timeout, return_when, loop)
def _release_waiter(waiter, *args):
if not waiter.done():
waiter.set_result(None)
async def wait_for(fut, timeout):
"""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.
If the task suppresses the cancellation and returns a value instead,
that value is returned.
This function is a coroutine.
"""
# The special case for timeout <= 0 is for the following case:
#
# async def test_waitfor():
# func_started = False
#
# async def func():
# nonlocal func_started
# func_started = True
#
# try:
# await asyncio.wait_for(func(), 0)
# except asyncio.TimeoutError:
# assert not func_started
# else:
# assert False
#
# asyncio.run(test_waitfor())
if timeout is not None and timeout <= 0:
fut = ensure_future(fut)
if fut.done():
return fut.result()
await _cancel_and_wait(fut)
try:
return fut.result()
except exceptions.CancelledError as exc:
raise TimeoutError from exc
async with timeouts.timeout(timeout):
return await fut
async def _wait(fs, timeout, return_when, loop):
"""Internal helper for wait().
The fs argument must be a collection of Futures.
"""
assert fs, 'Set of Futures is empty.'
waiter = loop.create_future()
timeout_handle = None
if timeout is not None:
timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
counter = len(fs)
def _on_completion(f):
nonlocal counter
counter -= 1
if (counter <= 0 or
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:
timeout_handle.cancel()
if not waiter.done():
waiter.set_result(None)
for f in fs:
f.add_done_callback(_on_completion)
try:
await waiter
finally:
if timeout_handle is not None:
timeout_handle.cancel()
for f in fs:
f.remove_done_callback(_on_completion)
done, pending = set(), set()
for f in fs:
if f.done():
done.add(f)
else:
pending.add(f)
return done, pending
async def _cancel_and_wait(fut):
"""Cancel the *fut* future or task and wait until it completes."""
loop = events.get_running_loop()
waiter = loop.create_future()
cb = functools.partial(_release_waiter, waiter)
fut.add_done_callback(cb)
try:
fut.cancel()
# We cannot wait on *fut* directly to make
# sure _cancel_and_wait itself is reliably cancellable.
await waiter
finally:
fut.remove_done_callback(cb)
class _AsCompletedIterator:
"""Iterator of awaitables representing tasks of asyncio.as_completed.
As an asynchronous iterator, iteration yields futures as they finish. As a
plain iterator, new coroutines are yielded that will return or raise the
result of the next underlying future to complete.
"""
def __init__(self, aws, timeout):
self._done = queues.Queue()
self._timeout_handle = None
loop = events.get_event_loop()
todo = {ensure_future(aw, loop=loop) for aw in set(aws)}
for f in todo:
f.add_done_callback(self._handle_completion)
if todo and timeout is not None:
self._timeout_handle = (
loop.call_later(timeout, self._handle_timeout)
)
self._todo = todo
self._todo_left = len(todo)
def __aiter__(self):
return self
def __iter__(self):
return self
async def __anext__(self):
if not self._todo_left:
raise StopAsyncIteration
assert self._todo_left > 0
self._todo_left -= 1
return await self._wait_for_one()
def __next__(self):
if not self._todo_left:
raise StopIteration
assert self._todo_left > 0
self._todo_left -= 1
return self._wait_for_one(resolve=True)
def _handle_timeout(self):
for f in self._todo:
f.remove_done_callback(self._handle_completion)
self._done.put_nowait(None) # Sentinel for _wait_for_one().
self._todo.clear() # Can't do todo.remove(f) in the loop.
def _handle_completion(self, f):
if not self._todo:
return # _handle_timeout() was here first.
self._todo.remove(f)
self._done.put_nowait(f)
if not self._todo and self._timeout_handle is not None:
self._timeout_handle.cancel()
async def _wait_for_one(self, resolve=False):
# Wait for the next future to be done and return it unless resolve is
# set, in which case return either the result of the future or raise
# an exception.
f = await self._done.get()
if f is None:
# Dummy value from _handle_timeout().
raise exceptions.TimeoutError
return f.result() if resolve else f
def as_completed(fs, *, timeout=None):
"""Create an iterator of awaitables or their results in completion order.
Run the supplied awaitables concurrently. The returned object can be
iterated to obtain the results of the awaitables as they finish.
The object returned can be iterated as an asynchronous iterator or a plain
iterator. When asynchronous iteration is used, the originally-supplied
awaitables are yielded if they are tasks or futures. This makes it easy to
correlate previously-scheduled tasks with their results:
ipv4_connect = create_task(open_connection("127.0.0.1", 80))
ipv6_connect = create_task(open_connection("::1", 80))
tasks = [ipv4_connect, ipv6_connect]
async for earliest_connect in as_completed(tasks):
# earliest_connect is done. The result can be obtained by
# awaiting it or calling earliest_connect.result()
reader, writer = await earliest_connect
if earliest_connect is ipv6_connect:
print("IPv6 connection established.")
else:
print("IPv4 connection established.")
During asynchronous iteration, implicitly-created tasks will be yielded for
supplied awaitables that aren't tasks or futures.
When used as a plain iterator, each iteration yields a new coroutine that
returns the result or raises the exception of the next completed awaitable.
This pattern is compatible with Python versions older than 3.13:
ipv4_connect = create_task(open_connection("127.0.0.1", 80))
ipv6_connect = create_task(open_connection("::1", 80))
tasks = [ipv4_connect, ipv6_connect]
for next_connect in as_completed(tasks):
# next_connect is not one of the original task objects. It must be
# awaited to obtain the result value or raise the exception of the
# awaitable that finishes next.
reader, writer = await next_connect
A TimeoutError is raised if the timeout occurs before all awaitables are
done. This is raised by the async for loop during asynchronous iteration or
by the coroutines yielded during plain iteration.
"""
if inspect.isawaitable(fs):
raise TypeError(
f"expects an iterable of awaitables, not {type(fs).__name__}"
)
return _AsCompletedIterator(fs, timeout)
@types.coroutine
def __sleep0():
"""Skip one event loop run cycle.
This is a private helper for 'asyncio.sleep()', used
when the 'delay' is set to 0. It uses a bare 'yield'
expression (which Task.__step knows how to handle)
instead of creating a Future object.
"""
yield
async def sleep(delay, result=None):
"""Coroutine that completes after a given time (in seconds)."""
if delay <= 0:
await __sleep0()
return result
if math.isnan(delay):
raise ValueError("Invalid delay: NaN (not a number)")
loop = events.get_running_loop()
future = loop.create_future()
h = loop.call_later(delay,
futures._set_result_unless_cancelled,
future, result)
try:
return await future
finally:
h.cancel()
def 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.
"""
if futures.isfuture(coro_or_future):
if loop is not None and loop is not futures._get_loop(coro_or_future):
raise ValueError('The future belongs to a different loop than '
'the one specified as the loop argument')
return coro_or_future
should_close = True
if not coroutines.iscoroutine(coro_or_future):
if inspect.isawaitable(coro_or_future):
async def _wrap_awaitable(awaitable):
return await awaitable
coro_or_future = _wrap_awaitable(coro_or_future)
should_close = False
else:
raise TypeError('An asyncio.Future, a coroutine or an awaitable '
'is required')
if loop is None:
loop = events.get_event_loop()
try:
return loop.create_task(coro_or_future)
except RuntimeError:
if should_close:
coro_or_future.close()
raise
class _GatheringFuture(futures.Future):
"""Helper for gather().
This overrides cancel() to cancel all the children and act more
like Task.cancel(), which doesn't immediately mark itself as
cancelled.
"""
def __init__(self, children, *, loop):
assert loop is not None
super().__init__(loop=loop)
self._children = children
self._cancel_requested = False
def cancel(self, msg=None):
if self.done():
return False
ret = False
for child in self._children:
if child.cancel(msg=msg):
ret = True
if ret:
# If any child tasks were actually cancelled, we should
# propagate the cancellation request regardless of
# *return_exceptions* argument. See issue 32684.
self._cancel_requested = True
return ret
def gather(*coros_or_futures, 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.
"""
if not coros_or_futures:
loop = events.get_event_loop()
outer = loop.create_future()
outer.set_result([])
return outer
def _done_callback(fut):
nonlocal nfinished
nfinished += 1
if outer is None or outer.done():
if not fut.cancelled():
# Mark exception retrieved.
fut.exception()
return
if not return_exceptions:
if fut.cancelled():
# Check if 'fut' is cancelled first, as
# 'fut.exception()' will *raise* a CancelledError
# instead of returning it.
exc = fut._make_cancelled_error()
outer.set_exception(exc)
return
else:
exc = fut.exception()
if exc is not None:
outer.set_exception(exc)
return
if nfinished == nfuts:
# All futures are done; create a list of results
# and set it to the 'outer' future.
results = []
for fut in children:
if fut.cancelled():
# Check if 'fut' is cancelled first, as 'fut.exception()'
# will *raise* a CancelledError instead of returning it.
# Also, since we're adding the exception return value
# to 'results' instead of raising it, don't bother
# setting __context__. This also lets us preserve
# calling '_make_cancelled_error()' at most once.
res = exceptions.CancelledError(
'' if fut._cancel_message is None else
fut._cancel_message)
else:
res = fut.exception()
if res is None:
res = fut.result()
results.append(res)
if outer._cancel_requested:
# If gather is being cancelled we must propagate the
# cancellation regardless of *return_exceptions* argument.
# See issue 32684.
exc = fut._make_cancelled_error()
outer.set_exception(exc)
else:
outer.set_result(results)
arg_to_fut = {}
children = []
nfuts = 0
nfinished = 0
done_futs = []
loop = None
outer = None # bpo-46672
for arg in coros_or_futures:
if arg not in arg_to_fut:
fut = ensure_future(arg, loop=loop)
if loop is None:
loop = futures._get_loop(fut)
if fut is not arg:
# 'arg' was not a Future, therefore, 'fut' is a new
# Future created specifically for 'arg'. Since the caller
# can't control it, disable the "destroy pending task"
# warning.
fut._log_destroy_pending = False
nfuts += 1
arg_to_fut[arg] = fut
if fut.done():
done_futs.append(fut)
else:
fut.add_done_callback(_done_callback)
else:
# There's a duplicate Future object in coros_or_futures.
fut = arg_to_fut[arg]
children.append(fut)
outer = _GatheringFuture(children, loop=loop)
# Run done callbacks after GatheringFuture created so any post-processing
# can be performed at this point
# optimization: in the special case that *all* futures finished eagerly,
# this will effectively complete the gather eagerly, with the last
# callback setting the result (or exception) on outer before returning it
for fut in done_futs:
_done_callback(fut)
return outer
def shield(arg):
"""Wait for a future, shielding it from cancellation.
The statement
task = asyncio.create_task(something())
res = await shield(task)
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:
task = asyncio.create_task(something())
try:
res = await shield(task)
except CancelledError:
res = None
Save a reference to tasks passed to this function, to avoid
a task disappearing mid-execution. The event loop only keeps
weak references to tasks. A task that isn't referenced elsewhere
may get garbage collected at any time, even before it's done.
"""
inner = ensure_future(arg)
if inner.done():
# Shortcut.
return inner
loop = futures._get_loop(inner)
outer = loop.create_future()
def _inner_done_callback(inner):
if outer.cancelled():
if not inner.cancelled():
# Mark inner's result as retrieved.
inner.exception()
return
if inner.cancelled():
outer.cancel()
else:
exc = inner.exception()
if exc is not None:
outer.set_exception(exc)
else:
outer.set_result(inner.result())
def _outer_done_callback(outer):
if not inner.done():
inner.remove_done_callback(_inner_done_callback)
inner.add_done_callback(_inner_done_callback)
outer.add_done_callback(_outer_done_callback)
return outer
def run_coroutine_threadsafe(coro, loop):
"""Submit a coroutine object to a given event loop.
Return a concurrent.futures.Future to access the result.
"""
if not coroutines.iscoroutine(coro):
raise TypeError('A coroutine object is required')
future = concurrent.futures.Future()
def callback():
try:
futures._chain_future(ensure_future(coro, loop=loop), future)
except (SystemExit, KeyboardInterrupt):
raise
except BaseException as exc:
if future.set_running_or_notify_cancel():
future.set_exception(exc)
raise
loop.call_soon_threadsafe(callback)
return future
def create_eager_task_factory(custom_task_constructor):
"""Create a function suitable for use as a task factory on an event-loop.
Example usage:
loop.set_task_factory(
asyncio.create_eager_task_factory(my_task_constructor))
Now, tasks created will be started immediately (rather than being first
scheduled to an event loop). The constructor argument can be any callable
that returns a Task-compatible object and has a signature compatible
with `Task.__init__`; it must have the `eager_start` keyword argument.
Most applications will use `Task` for `custom_task_constructor` and in
this case there's no need to call `create_eager_task_factory()`
directly. Instead the global `eager_task_factory` instance can be
used. E.g. `loop.set_task_factory(asyncio.eager_task_factory)`.
"""
def factory(loop, coro, *, name=None, context=None):
return custom_task_constructor(
coro, loop=loop, name=name, context=context, eager_start=True)
return factory
eager_task_factory = create_eager_task_factory(Task)
# Collectively these two sets hold references to the complete set of active
# tasks. Eagerly executed tasks use a faster regular set as an optimization
# but may graduate to a WeakSet if the task blocks on IO.
_scheduled_tasks = weakref.WeakSet()
_eager_tasks = set()
# Dictionary containing tasks that are currently active in
# all running event loops. {EventLoop: Task}
_current_tasks = {}
def _register_task(task):
"""Register an asyncio Task scheduled to run on an event loop."""
_scheduled_tasks.add(task)
def _register_eager_task(task):
"""Register an asyncio Task about to be eagerly executed."""
_eager_tasks.add(task)
def _enter_task(loop, task):
current_task = _current_tasks.get(loop)
if current_task is not None:
raise RuntimeError(f"Cannot enter into task {task!r} while another "
f"task {current_task!r} is being executed.")
_current_tasks[loop] = task
def _leave_task(loop, task):
current_task = _current_tasks.get(loop)
if current_task is not task:
raise RuntimeError(f"Leaving task {task!r} does not match "
f"the current task {current_task!r}.")
del _current_tasks[loop]
def _swap_current_task(loop, task):
prev_task = _current_tasks.get(loop)
if task is None:
del _current_tasks[loop]
else:
_current_tasks[loop] = task
return prev_task
def _unregister_task(task):
"""Unregister a completed, scheduled Task."""
_scheduled_tasks.discard(task)
def _unregister_eager_task(task):
"""Unregister a task which finished its first eager step."""
_eager_tasks.discard(task)
_py_current_task = current_task
_py_register_task = _register_task
_py_register_eager_task = _register_eager_task
_py_unregister_task = _unregister_task
_py_unregister_eager_task = _unregister_eager_task
_py_enter_task = _enter_task
_py_leave_task = _leave_task
_py_swap_current_task = _swap_current_task
_py_all_tasks = all_tasks
try:
from _asyncio import (_register_task, _register_eager_task,
_unregister_task, _unregister_eager_task,
_enter_task, _leave_task, _swap_current_task,
_scheduled_tasks, _eager_tasks, _current_tasks,
current_task, all_tasks)
except ImportError:
pass
else:
_c_current_task = current_task
_c_register_task = _register_task
_c_register_eager_task = _register_eager_task
_c_unregister_task = _unregister_task
_c_unregister_eager_task = _unregister_eager_task
_c_enter_task = _enter_task
_c_leave_task = _leave_task
_c_swap_current_task = _swap_current_task
_c_all_tasks = all_tasks
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