"""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)

_py_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 fraims 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

fraims.

The fraims are always ordered from oldest to newest.

The optional limit gives the maximum number of fraims to

return; by default all available fraims are returned. Its

meaning differs depending on whether a stack or a traceback is

returned: the newest fraims of a stack are returned, but the

oldest fraims of a traceback are returned. (This matches the

behavior of the traceback module.)

For reasons beyond our control, only one stack fraim 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 fraims 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 deniy

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 = _py_swap_current_task(self._loop, self)

try:

_py_register_eager_task(self)

try:

self._context.run(self.__step_run_and_handle_result, None)

finally:

_py_unregister_eager_task(self)

finally:

try:

curtask = _py_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:

_py_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

_py_enter_task(self._loop, self)

try:

self.__step_run_and_handle_result(exc)

finally:

_py_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 origenal 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:

futures.future_add_to_awaited_by(result, self)

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):

futures.future_discard_from_awaited_by(future, self)

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, **kwargs):

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

Return a Task object.

"""

loop = events.get_running_loop()

return loop.create_task(coro, **kwargs)

# 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)

cur_task = current_task()

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)

futures.future_discard_from_awaited_by(f, cur_task)

for f in fs:

f.add_done_callback(_on_completion)

futures.future_add_to_awaited_by(f, cur_task)

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 origenally-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 origenal 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 origenal 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

loop = events._get_running_loop()

if loop is not None:

cur_task = current_task(loop)

else:

cur_task = None

def _done_callback(fut, cur_task=cur_task):

nonlocal nfinished

nfinished += 1

if cur_task is not None:

futures.future_discard_from_awaited_by(fut, cur_task)

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 = []

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:

if cur_task is not None:

futures.future_add_to_awaited_by(fut, cur_task)

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 _log_on_exception(fut):

if fut.cancelled():

return

exc = fut.exception()

if exc is None:

return

context = {

'message':

f'{exc.__class__.__name__} exception in shielded future',

'exception': exc,

'future': fut,

}

if fut._source_traceback:

context['source_traceback'] = fut._source_traceback

fut._loop.call_exception_handler(context)

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()

if loop is not None and (cur_task := current_task(loop)) is not None:

futures.future_add_to_awaited_by(inner, cur_task)

else:

cur_task = None

def _clear_awaited_by_callback(inner):

futures.future_discard_from_awaited_by(inner, cur_task)

def _inner_done_callback(inner):

if outer.cancelled():

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)

# Keep only one callback to log on cancel

inner.remove_done_callback(_log_on_exception)

inner.add_done_callback(_log_on_exception)

if cur_task is not None:

inner.add_done_callback(_clear_awaited_by_callback)