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taskgroups.py
# Adapted with permission from the EdgeDB project; # license: PSFL. __all__ = ("TaskGroup",) from . import events from . import exceptions from . import tasks class TaskGroup: """Asynchronous context manager for managing groups of tasks. Example use: async with asyncio.TaskGroup() as group: task1 = group.create_task(some_coroutine(...)) task2 = group.create_task(other_coroutine(...)) print("Both tasks have completed now.") All tasks are awaited when the context manager exits. Any exceptions other than `asyncio.CancelledError` raised within a task will cancel all remaining tasks and wait for them to exit. The exceptions are then combined and raised as an `ExceptionGroup`. """ def __init__(self): self._entered = False self._exiting = False self._aborting = False self._loop = None self._parent_task = None self._parent_cancel_requested = False self._tasks = set() self._errors = [] self._base_error = None self._on_completed_fut = None def __repr__(self): info = [''] if self._tasks: info.append(f'tasks={len(self._tasks)}') if self._errors: info.append(f'errors={len(self._errors)}') if self._aborting: info.append('cancelling') elif self._entered: info.append('entered') info_str = ' '.join(info) return f'<TaskGroup{info_str}>' async def __aenter__(self): if self._entered: raise RuntimeError( f"TaskGroup {self!r} has already been entered") if self._loop is None: self._loop = events.get_running_loop() self._parent_task = tasks.current_task(self._loop) if self._parent_task is None: raise RuntimeError( f'TaskGroup {self!r} cannot determine the parent task') self._entered = True return self async def __aexit__(self, et, exc, tb): tb = None try: return await self._aexit(et, exc) finally: # Exceptions are heavy objects that can have object # cycles (bad for GC); let's not keep a reference to # a bunch of them. It would be nicer to use a try/finally # in __aexit__ directly but that introduced some diff noise self._parent_task = None self._errors = None self._base_error = None exc = None async def _aexit(self, et, exc): self._exiting = True if (exc is not None and self._is_base_error(exc) and self._base_error is None): self._base_error = exc propagate_cancellation_error = \ exc if et is exceptions.CancelledError else None if self._parent_cancel_requested: # If this flag is set we *must* call uncancel(). if self._parent_task.uncancel() == 0: # If there are no pending cancellations left, # don't propagate CancelledError. propagate_cancellation_error = None if et is not None: if not self._aborting: # Our parent task is being cancelled: # # async with TaskGroup() as g: # g.create_task(...) # await ... # <- CancelledError # # or there's an exception in "async with": # # async with TaskGroup() as g: # g.create_task(...) # 1 / 0 # self._abort() # We use while-loop here because "self._on_completed_fut" # can be cancelled multiple times if our parent task # is being cancelled repeatedly (or even once, when # our own cancellation is already in progress) while self._tasks: if self._on_completed_fut is None: self._on_completed_fut = self._loop.create_future() try: await self._on_completed_fut except exceptions.CancelledError as ex: if not self._aborting: # Our parent task is being cancelled: # # async def wrapper(): # async with TaskGroup() as g: # g.create_task(foo) # # "wrapper" is being cancelled while "foo" is # still running. propagate_cancellation_error = ex self._abort() self._on_completed_fut = None assert not self._tasks if self._base_error is not None: try: raise self._base_error finally: exc = None # Propagate CancelledError if there is one, except if there # are other errors -- those have priority. try: if propagate_cancellation_error and not self._errors: try: raise propagate_cancellation_error finally: exc = None finally: propagate_cancellation_error = None if et is not None and et is not exceptions.CancelledError: self._errors.append(exc) if self._errors: try: raise BaseExceptionGroup( 'unhandled errors in a TaskGroup', self._errors, ) from None finally: exc = None def create_task(self, coro, *, name=None, context=None): """Create a new task in this group and return it. Similar to `asyncio.create_task`. """ if not self._entered: raise RuntimeError(f"TaskGroup {self!r} has not been entered") if self._exiting and not self._tasks: raise RuntimeError(f"TaskGroup {self!r} is finished") if self._aborting: raise RuntimeError(f"TaskGroup {self!r} is shutting down") if context is None: task = self._loop.create_task(coro) else: task = self._loop.create_task(coro, context=context) tasks._set_task_name(task, name) # Always schedule the done callback even if the task is # already done (e.g. if the coro was able to complete eagerly), # otherwise if the task completes with an exception then it will cancel # the current task too early. gh-128550, gh-128588 self._tasks.add(task) task.add_done_callback(self._on_task_done) try: return task finally: # gh-128552: prevent a refcycle of # task.exception().__traceback__->TaskGroup.create_task->task del task # Since Python 3.8 Tasks propagate all exceptions correctly, # except for KeyboardInterrupt and SystemExit which are # still considered special. def _is_base_error(self, exc: BaseException) -> bool: assert isinstance(exc, BaseException) return isinstance(exc, (SystemExit, KeyboardInterrupt)) def _abort(self): self._aborting = True for t in self._tasks: if not t.done(): t.cancel() def _on_task_done(self, task): self._tasks.discard(task) if self._on_completed_fut is not None and not self._tasks: if not self._on_completed_fut.done(): self._on_completed_fut.set_result(True) if task.cancelled(): return exc = task.exception() if exc is None: return self._errors.append(exc) if self._is_base_error(exc) and self._base_error is None: self._base_error = exc if self._parent_task.done(): # Not sure if this case is possible, but we want to handle # it anyways. self._loop.call_exception_handler({ 'message': f'Task {task!r} has errored out but its parent ' f'task {self._parent_task} is already completed', 'exception': exc, 'task': task, }) return if not self._aborting and not self._parent_cancel_requested: # If parent task *is not* being cancelled, it means that we want # to manually cancel it to abort whatever is being run right now # in the TaskGroup. But we want to mark parent task as # "not cancelled" later in __aexit__. Example situation that # we need to handle: # # async def foo(): # try: # async with TaskGroup() as g: # g.create_task(crash_soon()) # await something # <- this needs to be canceled # # by the TaskGroup, e.g. # # foo() needs to be cancelled # except Exception: # # Ignore any exceptions raised in the TaskGroup # pass # await something_else # this line has to be called # # after TaskGroup is finished. self._abort() self._parent_cancel_requested = True self._parent_task.cancel()
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