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Editing: tasks.py
"""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', '_register_task', '_unregister_task', '_enter_task', '_leave_task', ) import concurrent.futures import contextvars import functools import inspect import itertools import types import warnings import weakref from types import GenericAlias from . import base_tasks from . import coroutines from . import events from . import exceptions from . import futures from .coroutines import _is_coroutine # 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() # Looping over a WeakSet (_all_tasks) isn't safe as it can be updated from another # thread while we do so. Therefore we cast it to list prior to filtering. The list # cast itself requires iteration, so we repeat it several times ignoring # RuntimeErrors (which are not very likely to occur). See issues 34970 and 36607 for # details. i = 0 while True: try: tasks = list(_all_tasks) except RuntimeError: i += 1 if i >= 1000: raise else: break return {t for t in tasks if futures._get_loop(t) is loop and not t.done()} def _set_task_name(task, name): if name is not None: try: set_name = task.set_name except AttributeError: warnings.warn("Task.set_name() was added in Python 3.8, " "the method support will be mandatory for third-party " "task implementations since 3.13.", DeprecationWarning, stacklevel=3) else: set_name(name) 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): 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 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_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 return self._num_cancels_requested 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 coro = self._coro self._fut_waiter = None _enter_task(self._loop, self) # Call either coro.throw(exc) or coro.send(None). 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: _leave_task(self._loop, self) 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) else: task = loop.create_task(coro, context=context) _set_task_name(task, name) 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. 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. """ 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. This function is a coroutine. """ loop = events.get_running_loop() if timeout is None: return await fut if timeout <= 0: fut = ensure_future(fut, loop=loop) if fut.done(): return fut.result() await _cancel_and_wait(fut, loop=loop) try: return fut.result() except exceptions.CancelledError as exc: raise exceptions.TimeoutError() from exc 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) try: # wait until the future completes or the timeout try: await waiter except exceptions.CancelledError: if fut.done(): return fut.result() else: fut.remove_done_callback(cb) # We must ensure that the task is not running # after wait_for() returns. # See https://bugs.python.org/issue32751 await _cancel_and_wait(fut, loop=loop) raise if fut.done(): return fut.result() else: fut.remove_done_callback(cb) # We must ensure that the task is not running # after wait_for() returns. # See https://bugs.python.org/issue32751 await _cancel_and_wait(fut, loop=loop) # In case task cancellation failed with some # exception, we should re-raise it # See https://bugs.python.org/issue40607 try: return fut.result() except exceptions.CancelledError as exc: raise exceptions.TimeoutError() from exc finally: timeout_handle.cancel() 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, loop): """Cancel the *fut* future or task and wait until it completes.""" 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) # This is *not* a @coroutine! It is just an iterator (yielding Futures). def as_completed(fs, *, 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. """ if futures.isfuture(fs) or coroutines.iscoroutine(fs): raise TypeError(f"expect an iterable of futures, not {type(fs).__name__}") from .queues import Queue # Import here to avoid circular import problem. done = Queue() loop = events._get_event_loop() todo = {ensure_future(f, loop=loop) for f in set(fs)} timeout_handle = None def _on_timeout(): for f in todo: 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. def _on_completion(f): if not todo: return # _on_timeout() was here first. todo.remove(f) done.put_nowait(f) if not todo and timeout_handle is not None: timeout_handle.cancel() async def _wait_for_one(): f = await done.get() if f is None: # Dummy value from _on_timeout(). raise exceptions.TimeoutError return f.result() # May raise f.exception(). for f in todo: 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)): yield _wait_for_one() @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 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. """ return _ensure_future(coro_or_future, loop=loop) def _ensure_future(coro_or_future, *, loop=None): 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 called_wrap_awaitable = False if not coroutines.iscoroutine(coro_or_future): if inspect.isawaitable(coro_or_future): coro_or_future = _wrap_awaitable(coro_or_future) called_wrap_awaitable = True else: raise TypeError('An asyncio.Future, a coroutine or an awaitable ' 'is required') if loop is None: loop = events._get_event_loop(stacklevel=4) try: return loop.create_task(coro_or_future) except RuntimeError: if not called_wrap_awaitable: coro_or_future.close() raise @types.coroutine def _wrap_awaitable(awaitable): """Helper for asyncio.ensure_future(). Wraps awaitable (an object with __await__) into a coroutine that will later be wrapped in a Task by ensure_future(). """ return (yield from awaitable.__await__()) _wrap_awaitable._is_coroutine = _is_coroutine 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 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 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) 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 # WeakSet containing all alive tasks. _all_tasks = weakref.WeakSet() # Dictionary containing tasks that are currently active in # all running event loops. {EventLoop: Task} _current_tasks = {} def _register_task(task): """Register a new task in asyncio as executed by loop.""" _all_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 _unregister_task(task): """Unregister a task.""" _all_tasks.discard(task) _py_register_task = _register_task _py_unregister_task = _unregister_task _py_enter_task = _enter_task _py_leave_task = _leave_task try: from _asyncio import (_register_task, _unregister_task, _enter_task, _leave_task, _all_tasks, _current_tasks) except ImportError: pass else: _c_register_task = _register_task _c_unregister_task = _unregister_task _c_enter_task = _enter_task _c_leave_task = _leave_task
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