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_base.py
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# Copyright 2009 Brian Quinlan. All Rights Reserved. # Licensed to PSF under a Contributor Agreement. __author__ = 'Brian Quinlan (brian@sweetapp.com)' import collections import logging import threading import time FIRST_COMPLETED = 'FIRST_COMPLETED' FIRST_EXCEPTION = 'FIRST_EXCEPTION' ALL_COMPLETED = 'ALL_COMPLETED' _AS_COMPLETED = '_AS_COMPLETED' # Possible future states (for internal use by the futures package). PENDING = 'PENDING' RUNNING = 'RUNNING' # The future was cancelled by the user... CANCELLED = 'CANCELLED' # ...and _Waiter.add_cancelled() was called by a worker. CANCELLED_AND_NOTIFIED = 'CANCELLED_AND_NOTIFIED' FINISHED = 'FINISHED' _FUTURE_STATES = [ PENDING, RUNNING, CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED ] _STATE_TO_DESCRIPTION_MAP = { PENDING: "pending", RUNNING: "running", CANCELLED: "cancelled", CANCELLED_AND_NOTIFIED: "cancelled", FINISHED: "finished" } # Logger for internal use by the futures package. LOGGER = logging.getLogger("concurrent.futures") class Error(Exception): """Base class for all future-related exceptions.""" pass class CancelledError(Error): """The Future was cancelled.""" pass class TimeoutError(Error): """The operation exceeded the given deadline.""" pass class _Waiter(object): """Provides the event that wait() and as_completed() block on.""" def __init__(self): self.event = threading.Event() self.finished_futures = [] def add_result(self, future): self.finished_futures.append(future) def add_exception(self, future): self.finished_futures.append(future) def add_cancelled(self, future): self.finished_futures.append(future) class _AsCompletedWaiter(_Waiter): """Used by as_completed().""" def __init__(self): super(_AsCompletedWaiter, self).__init__() self.lock = threading.Lock() def add_result(self, future): with self.lock: super(_AsCompletedWaiter, self).add_result(future) self.event.set() def add_exception(self, future): with self.lock: super(_AsCompletedWaiter, self).add_exception(future) self.event.set() def add_cancelled(self, future): with self.lock: super(_AsCompletedWaiter, self).add_cancelled(future) self.event.set() class _FirstCompletedWaiter(_Waiter): """Used by wait(return_when=FIRST_COMPLETED).""" def add_result(self, future): super().add_result(future) self.event.set() def add_exception(self, future): super().add_exception(future) self.event.set() def add_cancelled(self, future): super().add_cancelled(future) self.event.set() class _AllCompletedWaiter(_Waiter): """Used by wait(return_when=FIRST_EXCEPTION and ALL_COMPLETED).""" def __init__(self, num_pending_calls, stop_on_exception): self.num_pending_calls = num_pending_calls self.stop_on_exception = stop_on_exception self.lock = threading.Lock() super().__init__() def _decrement_pending_calls(self): with self.lock: self.num_pending_calls -= 1 if not self.num_pending_calls: self.event.set() def add_result(self, future): super().add_result(future) self._decrement_pending_calls() def add_exception(self, future): super().add_exception(future) if self.stop_on_exception: self.event.set() else: self._decrement_pending_calls() def add_cancelled(self, future): super().add_cancelled(future) self._decrement_pending_calls() class _AcquireFutures(object): """A context manager that does an ordered acquire of Future conditions.""" def __init__(self, futures): self.futures = sorted(futures, key=id) def __enter__(self): for future in self.futures: future._condition.acquire() def __exit__(self, *args): for future in self.futures: future._condition.release() def _create_and_install_waiters(fs, return_when): if return_when == _AS_COMPLETED: waiter = _AsCompletedWaiter() elif return_when == FIRST_COMPLETED: waiter = _FirstCompletedWaiter() else: pending_count = sum( f._state not in [CANCELLED_AND_NOTIFIED, FINISHED] for f in fs) if return_when == FIRST_EXCEPTION: waiter = _AllCompletedWaiter(pending_count, stop_on_exception=True) elif return_when == ALL_COMPLETED: waiter = _AllCompletedWaiter(pending_count, stop_on_exception=False) else: raise ValueError("Invalid return condition: %r" % return_when) for f in fs: f._waiters.append(waiter) return waiter def _yield_finished_futures(fs, waiter, ref_collect): """ Iterate on the list *fs*, yielding finished futures one by one in reverse order. Before yielding a future, *waiter* is removed from its waiters and the future is removed from each set in the collection of sets *ref_collect*. The aim of this function is to avoid keeping stale references after the future is yielded and before the iterator resumes. """ while fs: f = fs[-1] for futures_set in ref_collect: futures_set.remove(f) with f._condition: f._waiters.remove(waiter) del f # Careful not to keep a reference to the popped value yield fs.pop() def as_completed(fs, timeout=None): """An iterator over the given futures that yields each as it completes. Args: fs: The sequence of Futures (possibly created by different Executors) to iterate over. timeout: The maximum number of seconds to wait. If None, then there is no limit on the wait time. Returns: An iterator that yields the given Futures as they complete (finished or cancelled). If any given Futures are duplicated, they will be returned once. Raises: TimeoutError: If the entire result iterator could not be generated before the given timeout. """ if timeout is not None: end_time = timeout + time.monotonic() fs = set(fs) total_futures = len(fs) with _AcquireFutures(fs): finished = set( f for f in fs if f._state in [CANCELLED_AND_NOTIFIED, FINISHED]) pending = fs - finished waiter = _create_and_install_waiters(fs, _AS_COMPLETED) finished = list(finished) try: yield from _yield_finished_futures(finished, waiter, ref_collect=(fs,)) while pending: if timeout is None: wait_timeout = None else: wait_timeout = end_time - time.monotonic() if wait_timeout < 0: raise TimeoutError( '%d (of %d) futures unfinished' % ( len(pending), total_futures)) waiter.event.wait(wait_timeout) with waiter.lock: finished = waiter.finished_futures waiter.finished_futures = [] waiter.event.clear() # reverse to keep finishing order finished.reverse() yield from _yield_finished_futures(finished, waiter, ref_collect=(fs, pending)) finally: # Remove waiter from unfinished futures for f in fs: with f._condition: f._waiters.remove(waiter) DoneAndNotDoneFutures = collections.namedtuple( 'DoneAndNotDoneFutures', 'done not_done') def wait(fs, timeout=None, return_when=ALL_COMPLETED): """Wait for the futures in the given sequence to complete. Args: fs: The sequence of Futures (possibly created by different Executors) to wait upon. timeout: The maximum number of seconds to wait. If None, then there is no limit on the wait time. return_when: Indicates when this function should return. The options are: FIRST_COMPLETED - Return when any future finishes or is cancelled. FIRST_EXCEPTION - Return when any future finishes by raising an exception. If no future raises an exception then it is equivalent to ALL_COMPLETED. ALL_COMPLETED - Return when all futures finish or are cancelled. Returns: A named 2-tuple of sets. The first set, named 'done', contains the futures that completed (is finished or cancelled) before the wait completed. The second set, named 'not_done', contains uncompleted futures. """ with _AcquireFutures(fs): done = set(f for f in fs if f._state in [CANCELLED_AND_NOTIFIED, FINISHED]) not_done = set(fs) - done if (return_when == FIRST_COMPLETED) and done: return DoneAndNotDoneFutures(done, not_done) elif (return_when == FIRST_EXCEPTION) and done: if any(f for f in done if not f.cancelled() and f.exception() is not None): return DoneAndNotDoneFutures(done, not_done) if len(done) == len(fs): return DoneAndNotDoneFutures(done, not_done) waiter = _create_and_install_waiters(fs, return_when) waiter.event.wait(timeout) for f in fs: with f._condition: f._waiters.remove(waiter) done.update(waiter.finished_futures) return DoneAndNotDoneFutures(done, set(fs) - done) class Future(object): """Represents the result of an asynchronous computation.""" def __init__(self): """Initializes the future. Should not be called by clients.""" self._condition = threading.Condition() self._state = PENDING self._result = None self._exception = None self._waiters = [] self._done_callbacks = [] def _invoke_callbacks(self): for callback in self._done_callbacks: try: callback(self) except Exception: LOGGER.exception('exception calling callback for %r', self) def __repr__(self): with self._condition: if self._state == FINISHED: if self._exception: return '<%s at %#x state=%s raised %s>' % ( self.__class__.__name__, id(self), _STATE_TO_DESCRIPTION_MAP[self._state], self._exception.__class__.__name__) else: return '<%s at %#x state=%s returned %s>' % ( self.__class__.__name__, id(self), _STATE_TO_DESCRIPTION_MAP[self._state], self._result.__class__.__name__) return '<%s at %#x state=%s>' % ( self.__class__.__name__, id(self), _STATE_TO_DESCRIPTION_MAP[self._state]) def cancel(self): """Cancel the future if possible. Returns True if the future was cancelled, False otherwise. A future cannot be cancelled if it is running or has already completed. """ with self._condition: if self._state in [RUNNING, FINISHED]: return False if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]: return True self._state = CANCELLED self._condition.notify_all() self._invoke_callbacks() return True def cancelled(self): """Return True if the future was cancelled.""" with self._condition: return self._state in [CANCELLED, CANCELLED_AND_NOTIFIED] def running(self): """Return True if the future is currently executing.""" with self._condition: return self._state == RUNNING def done(self): """Return True of the future was cancelled or finished executing.""" with self._condition: return self._state in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED] def __get_result(self): if self._exception: raise self._exception else: return self._result def add_done_callback(self, fn): """Attaches a callable that will be called when the future finishes. Args: fn: A callable that will be called with this future as its only argument when the future completes or is cancelled. The callable will always be called by a thread in the same process in which it was added. If the future has already completed or been cancelled then the callable will be called immediately. These callables are called in the order that they were added. """ with self._condition: if self._state not in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED]: self._done_callbacks.append(fn) return try: fn(self) except Exception: LOGGER.exception('exception calling callback for %r', self) def result(self, timeout=None): """Return the result of the call that the future represents. Args: timeout: The number of seconds to wait for the result if the future isn't done. If None, then there is no limit on the wait time. Returns: The result of the call that the future represents. Raises: CancelledError: If the future was cancelled. TimeoutError: If the future didn't finish executing before the given timeout. Exception: If the call raised then that exception will be raised. """ with self._condition: if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]: raise CancelledError() elif self._state == FINISHED: return self.__get_result() self._condition.wait(timeout) if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]: raise CancelledError() elif self._state == FINISHED: return self.__get_result() else: raise TimeoutError() def exception(self, timeout=None): """Return the exception raised by the call that the future represents. Args: timeout: The number of seconds to wait for the exception if the future isn't done. If None, then there is no limit on the wait time. Returns: The exception raised by the call that the future represents or None if the call completed without raising. Raises: CancelledError: If the future was cancelled. TimeoutError: If the future didn't finish executing before the given timeout. """ with self._condition: if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]: raise CancelledError() elif self._state == FINISHED: return self._exception self._condition.wait(timeout) if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]: raise CancelledError() elif self._state == FINISHED: return self._exception else: raise TimeoutError() # The following methods should only be used by Executors and in tests. def set_running_or_notify_cancel(self): """Mark the future as running or process any cancel notifications. Should only be used by Executor implementations and unit tests. If the future has been cancelled (cancel() was called and returned True) then any threads waiting on the future completing (though calls to as_completed() or wait()) are notified and False is returned. If the future was not cancelled then it is put in the running state (future calls to running() will return True) and True is returned. This method should be called by Executor implementations before executing the work associated with this future. If this method returns False then the work should not be executed. Returns: False if the Future was cancelled, True otherwise. Raises: RuntimeError: if this method was already called or if set_result() or set_exception() was called. """ with self._condition: if self._state == CANCELLED: self._state = CANCELLED_AND_NOTIFIED for waiter in self._waiters: waiter.add_cancelled(self) # self._condition.notify_all() is not necessary because # self.cancel() triggers a notification. return False elif self._state == PENDING: self._state = RUNNING return True else: LOGGER.critical('Future %s in unexpected state: %s', id(self), self._state) raise RuntimeError('Future in unexpected state') def set_result(self, result): """Sets the return value of work associated with the future. Should only be used by Executor implementations and unit tests. """ with self._condition: self._result = result self._state = FINISHED for waiter in self._waiters: waiter.add_result(self) self._condition.notify_all() self._invoke_callbacks() def set_exception(self, exception): """Sets the result of the future as being the given exception. Should only be used by Executor implementations and unit tests. """ with self._condition: self._exception = exception self._state = FINISHED for waiter in self._waiters: waiter.add_exception(self) self._condition.notify_all() self._invoke_callbacks() class Executor(object): """This is an abstract base class for concrete asynchronous executors.""" def submit(*args, **kwargs): """Submits a callable to be executed with the given arguments. Schedules the callable to be executed as fn(*args, **kwargs) and returns a Future instance representing the execution of the callable. Returns: A Future representing the given call. """ if len(args) >= 2: pass elif not args: raise TypeError("descriptor 'submit' of 'Executor' object " "needs an argument") elif 'fn' not in kwargs: raise TypeError('submit expected at least 1 positional argument, ' 'got %d' % (len(args)-1)) raise NotImplementedError() def map(self, fn, *iterables, timeout=None, chunksize=1): """Returns an iterator equivalent to map(fn, iter). Args: fn: A callable that will take as many arguments as there are passed iterables. timeout: The maximum number of seconds to wait. If None, then there is no limit on the wait time. chunksize: The size of the chunks the iterable will be broken into before being passed to a child process. This argument is only used by ProcessPoolExecutor; it is ignored by ThreadPoolExecutor. Returns: An iterator equivalent to: map(func, *iterables) but the calls may be evaluated out-of-order. Raises: TimeoutError: If the entire result iterator could not be generated before the given timeout. Exception: If fn(*args) raises for any values. """ if timeout is not None: end_time = timeout + time.monotonic() fs = [self.submit(fn, *args) for args in zip(*iterables)] # Yield must be hidden in closure so that the futures are submitted # before the first iterator value is required. def result_iterator(): try: # reverse to keep finishing order fs.reverse() while fs: # Careful not to keep a reference to the popped future if timeout is None: yield fs.pop().result() else: yield fs.pop().result(end_time - time.monotonic()) finally: for future in fs: future.cancel() return result_iterator() def shutdown(self, wait=True): """Clean-up the resources associated with the Executor. It is safe to call this method several times. Otherwise, no other methods can be called after this one. Args: wait: If True then shutdown will not return until all running futures have finished executing and the resources used by the executor have been reclaimed. """ pass def __enter__(self): return self def __exit__(self, exc_type, exc_val, exc_tb): self.shutdown(wait=True) return False class BrokenExecutor(RuntimeError): """ Raised when a executor has become non-functional after a severe failure. """