Edit: tasks.py
"""Support for tasks, coroutines and the scheduler.""" __all__ = ['Task', 'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED', 'wait', 'wait_for', 'as_completed', 'sleep', 'async', 'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe', ] import concurrent.futures import functools import inspect import warnings import weakref from . import base_tasks from . import compat from . import coroutines from . import events from . import 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 # must be _wakeup(). # Weak set containing all tasks alive. _all_tasks = weakref.WeakSet() # Dictionary containing tasks that are currently active in # all running event loops. {EventLoop: Task} _current_tasks = {} # If False, don't log a message if the task is destroyed whereas its # status is still pending _log_destroy_pending = True @classmethod 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. """ if loop is None: loop = events.get_event_loop() return cls._current_tasks.get(loop) @classmethod 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. """ if loop is None: 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._coro = coro self._fut_waiter = None 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 compat.PY34: 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) futures.Future.__del__(self) def _repr_info(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 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): """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). """ self._log_traceback = False if self.done(): return 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. return True # It must be the case that self._step is already scheduled. self._must_cancel = True return True def _step(self, exc=None): assert not self.done(), \ '_step(): already done: {!r}, {!r}'.format(self, exc) if self._must_cancel: if not isinstance(exc, futures.CancelledError): exc = futures.CancelledError() self._must_cancel = False coro = self._coro self._fut_waiter = None self.__class__._current_tasks[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 self.set_exception(futures.CancelledError()) else: self.set_result(exc.value) except futures.CancelledError: super().cancel() # I.e., Future.cancel(self). except Exception as exc: self.set_exception(exc) except BaseException as exc: self.set_exception(exc) raise else: blocking = getattr(result, '_asyncio_future_blocking', None) if blocking is not None: # Yielded Future must come from Future.__iter__(). if result._loop is not self._loop: self._loop.call_soon( self._step, RuntimeError( 'Task {!r} got Future {!r} attached to a ' 'different loop'.format(self, result))) elif blocking: if result is self: self._loop.call_soon( self._step, RuntimeError( 'Task cannot await on itself: {!r}'.format( self))) else: result._asyncio_future_blocking = False result.add_done_callback(self._wakeup) self._fut_waiter = result if self._must_cancel: if self._fut_waiter.cancel(): self._must_cancel = False else: self._loop.call_soon( self._step, RuntimeError( 'yield was used instead of yield from ' 'in task {!r} with {!r}'.format(self, result))) elif result is None: # 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. self._loop.call_soon( self._step, RuntimeError( 'yield was used instead of yield from for ' 'generator in task {!r} with {!r}'.format( self, result))) else: # Yielding something else is an error. self._loop.call_soon( self._step, RuntimeError( 'Task got bad yield: {!r}'.format(result))) finally: self.__class__._current_tasks.pop(self._loop) self = None # Needed to break cycles when an exception occurs. def _wakeup(self, future): try: future.result() except Exception 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 # 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 @coroutine 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). Usage: 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__) if not fs: 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)) if loop is None: 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): if not waiter.done(): waiter.set_result(None) @coroutine 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. """ if loop is None: loop = events.get_event_loop() if timeout is None: return (yield from fut) 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: yield from waiter except futures.CancelledError: fut.remove_done_callback(cb) fut.cancel() raise if fut.done(): return fut.result() else: fut.remove_done_callback(cb) fut.cancel() raise futures.TimeoutError() finally: timeout_handle.cancel() @coroutine 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 = 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: yield from waiter finally: if timeout_handle is not None: timeout_handle.cancel() done, pending = set(), set() for f in fs: f.remove_done_callback(_on_completion) if f.done(): done.add(f) else: pending.add(f) return done, pending # 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. # Use result. 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. done = Queue(loop=loop) 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() @coroutine def _wait_for_one(): f = yield from done.get() if f is None: # Dummy value from _on_timeout(). raise futures.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() @coroutine def sleep(delay, result=None, *, loop=None): """Coroutine that completes after a given time (in seconds).""" if delay == 0: yield return result if loop is None: loop = events.get_event_loop() future = loop.create_future() h = future._loop.call_later(delay, futures._set_result_unless_cancelled, future, result) try: return (yield from future) finally: h.cancel() 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()", DeprecationWarning, stacklevel=2) return ensure_future(coro_or_future, loop=loop) # Silence DeprecationWarning: globals()['async'] = async_ async_.__name__ = 'async' del async_ 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 coro_or_future._loop: raise ValueError('loop argument must agree with Future') return coro_or_future elif coroutines.iscoroutine(coro_or_future): if loop is None: loop = events.get_event_loop() task = loop.create_task(coro_or_future) if task._source_traceback: del task._source_traceback[-1] return task elif compat.PY35 and inspect.isawaitable(coro_or_future): return ensure_future(_wrap_awaitable(coro_or_future), loop=loop) else: raise TypeError('An asyncio.Future, a coroutine or an awaitable is ' 'required') @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__()) 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=None): super().__init__(loop=loop) self._children = children self._cancel_requested = False def cancel(self): if self.done(): return False ret = False for child in self._children: if child.cancel(): 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, loop=None, return_exceptions=False): """Return a future aggregating results from the given coroutines or 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 not coros_or_futures: if loop is None: loop = events.get_event_loop() outer = loop.create_future() outer.set_result([]) return outer arg_to_fut = {} for arg in set(coros_or_futures): if not futures.isfuture(arg): fut = ensure_future(arg, loop=loop) if loop is None: loop = fut._loop # The caller cannot control this future, the "destroy pending task" # warning should not be emitted. fut._log_destroy_pending = False else: fut = arg if loop is None: loop = fut._loop elif fut._loop is not loop: raise ValueError("futures are tied to different event loops") arg_to_fut[arg] = fut children = [arg_to_fut[arg] for arg in coros_or_futures] nchildren = len(children) outer = _GatheringFuture(children, loop=loop) nfinished = 0 results = [None] * nchildren def _done_callback(i, fut): nonlocal nfinished if outer.done(): if not fut.cancelled(): # Mark exception retrieved. fut.exception() return if fut.cancelled(): res = futures.CancelledError() if not return_exceptions: outer.set_exception(res) return elif fut._exception is not None: res = fut.exception() # Mark exception retrieved. if not return_exceptions: outer.set_exception(res) return else: res = fut._result results[i] = res nfinished += 1 if nfinished == nchildren: if outer._cancel_requested: outer.set_exception(futures.CancelledError()) else: outer.set_result(results) for i, fut in enumerate(children): fut.add_done_callback(functools.partial(_done_callback, i)) return outer def shield(arg, *, loop=None): """Wait for a future, shielding it from cancellation. The statement res = yield from shield(something()) is exactly equivalent to the statement res = yield from something() *except* that if the coroutine containing it is cancelled, the task running in something() is not cancelled. From the POV of something(), the cancellation did not happen. But its caller is still cancelled, so the yield-from expression still raises CancelledError. Note: If something() is cancelled by other means this will still cancel shield(). If you want to completely ignore cancellation (not recommended) you can combine shield() with a try/except clause, as follows: try: res = yield from shield(something()) except CancelledError: res = None """ inner = ensure_future(arg, loop=loop) if inner.done(): # Shortcut. return inner loop = inner._loop outer = loop.create_future() def _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()) inner.add_done_callback(_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 Exception as exc: if future.set_running_or_notify_cancel(): future.set_exception(exc) raise loop.call_soon_threadsafe(callback) return future
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