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staggered.py
"""Support for running coroutines in parallel with staggered start times.""" __all__ = 'staggered_race', import contextlib import typing from . import events from . import exceptions as exceptions_mod from . import locks from . import tasks async def staggered_race( coro_fns: typing.Iterable[typing.Callable[[], typing.Awaitable]], delay: typing.Optional[float], *, loop: events.AbstractEventLoop = None, ) -> typing.Tuple[ typing.Any, typing.Optional[int], typing.List[typing.Optional[Exception]] ]: """Run coroutines with staggered start times and take the first to finish. This method takes an iterable of coroutine functions. The first one is started immediately. From then on, whenever the immediately preceding one fails (raises an exception), or when *delay* seconds has passed, the next coroutine is started. This continues until one of the coroutines complete successfully, in which case all others are cancelled, or until all coroutines fail. The coroutines provided should be well-behaved in the following way: * They should only ``return`` if completed successfully. * They should always raise an exception if they did not complete successfully. In particular, if they handle cancellation, they should probably reraise, like this:: try: # do work except asyncio.CancelledError: # undo partially completed work raise Args: coro_fns: an iterable of coroutine functions, i.e. callables that return a coroutine object when called. Use ``functools.partial`` or lambdas to pass arguments. delay: amount of time, in seconds, between starting coroutines. If ``None``, the coroutines will run sequentially. loop: the event loop to use. Returns: tuple *(winner_result, winner_index, exceptions)* where - *winner_result*: the result of the winning coroutine, or ``None`` if no coroutines won. - *winner_index*: the index of the winning coroutine in ``coro_fns``, or ``None`` if no coroutines won. If the winning coroutine may return None on success, *winner_index* can be used to definitively determine whether any coroutine won. - *exceptions*: list of exceptions returned by the coroutines. ``len(exceptions)`` is equal to the number of coroutines actually started, and the order is the same as in ``coro_fns``. The winning coroutine's entry is ``None``. """ # TODO: when we have aiter() and anext(), allow async iterables in coro_fns. loop = loop or events.get_running_loop() enum_coro_fns = enumerate(coro_fns) winner_result = None winner_index = None exceptions = [] running_tasks = [] async def run_one_coro( previous_failed: typing.Optional[locks.Event]) -> None: # Wait for the previous task to finish, or for delay seconds if previous_failed is not None: with contextlib.suppress(exceptions_mod.TimeoutError): # Use asyncio.wait_for() instead of asyncio.wait() here, so # that if we get cancelled at this point, Event.wait() is also # cancelled, otherwise there will be a "Task destroyed but it is # pending" later. await tasks.wait_for(previous_failed.wait(), delay) # Get the next coroutine to run try: this_index, coro_fn = next(enum_coro_fns) except StopIteration: return # Start task that will run the next coroutine this_failed = locks.Event() next_task = loop.create_task(run_one_coro(this_failed)) running_tasks.append(next_task) assert len(running_tasks) == this_index + 2 # Prepare place to put this coroutine's exceptions if not won exceptions.append(None) assert len(exceptions) == this_index + 1 try: result = await coro_fn() except (SystemExit, KeyboardInterrupt): raise except BaseException as e: exceptions[this_index] = e this_failed.set() # Kickstart the next coroutine else: # Store winner's results nonlocal winner_index, winner_result assert winner_index is None winner_index = this_index winner_result = result # Cancel all other tasks. We take care to not cancel the current # task as well. If we do so, then since there is no `await` after # here and CancelledError are usually thrown at one, we will # encounter a curious corner case where the current task will end # up as done() == True, cancelled() == False, exception() == # asyncio.CancelledError. This behavior is specified in # https://bugs.python.org/issue30048 for i, t in enumerate(running_tasks): if i != this_index: t.cancel() first_task = loop.create_task(run_one_coro(None)) running_tasks.append(first_task) try: # Wait for a growing list of tasks to all finish: poor man's version of # curio's TaskGroup or trio's nursery done_count = 0 while done_count != len(running_tasks): done, _ = await tasks.wait(running_tasks) done_count = len(done) # If run_one_coro raises an unhandled exception, it's probably a # programming error, and I want to see it. if __debug__: for d in done: if d.done() and not d.cancelled() and d.exception(): raise d.exception() return winner_result, winner_index, exceptions finally: # Make sure no tasks are left running if we leave this function for t in running_tasks: t.cancel()
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