Editing: futures.py
"""A Future class similar to the one in PEP 3148.""" __all__ = ['CancelledError', 'TimeoutError', 'InvalidStateError', 'Future', 'wrap_future', 'isfuture'] import concurrent.futures import logging import sys import traceback from . import base_futures from . import compat from . import events CancelledError = base_futures.CancelledError InvalidStateError = base_futures.InvalidStateError TimeoutError = base_futures.TimeoutError isfuture = base_futures.isfuture _PENDING = base_futures._PENDING _CANCELLED = base_futures._CANCELLED _FINISHED = base_futures._FINISHED STACK_DEBUG = logging.DEBUG - 1 # heavy-duty debugging class _TracebackLogger: """Helper to log a traceback upon destruction if not cleared. This solves a nasty problem with Futures and Tasks that have an exception set: if nobody asks for the exception, the exception is never logged. This violates the Zen of Python: 'Errors should never pass silently. Unless explicitly silenced.' However, we don't want to log the exception as soon as set_exception() is called: if the calling code is written properly, it will get the exception and handle it properly. But we *do* want to log it if result() or exception() was never called -- otherwise developers waste a lot of time wondering why their buggy code fails silently. An earlier attempt added a __del__() method to the Future class itself, but this backfired because the presence of __del__() prevents garbage collection from breaking cycles. A way out of this catch-22 is to avoid having a __del__() method on the Future class itself, but instead to have a reference to a helper object with a __del__() method that logs the traceback, where we ensure that the helper object doesn't participate in cycles, and only the Future has a reference to it. The helper object is added when set_exception() is called. When the Future is collected, and the helper is present, the helper object is also collected, and its __del__() method will log the traceback. When the Future's result() or exception() method is called (and a helper object is present), it removes the helper object, after calling its clear() method to prevent it from logging. One downside is that we do a fair amount of work to extract the traceback from the exception, even when it is never logged. It would seem cheaper to just store the exception object, but that references the traceback, which references stack frames, which may reference the Future, which references the _TracebackLogger, and then the _TracebackLogger would be included in a cycle, which is what we're trying to avoid! As an optimization, we don't immediately format the exception; we only do the work when activate() is called, which call is delayed until after all the Future's callbacks have run. Since usually a Future has at least one callback (typically set by 'yield from') and usually that callback extracts the callback, thereby removing the need to format the exception. PS. I don't claim credit for this solution. I first heard of it in a discussion about closing files when they are collected. """ __slots__ = ('loop', 'source_traceback', 'exc', 'tb') def __init__(self, future, exc): self.loop = future._loop self.source_traceback = future._source_traceback self.exc = exc self.tb = None def activate(self): exc = self.exc if exc is not None: self.exc = None self.tb = traceback.format_exception(exc.__class__, exc, exc.__traceback__) def clear(self): self.exc = None self.tb = None def __del__(self): if self.tb: msg = 'Future/Task exception was never retrieved\n' if self.source_traceback: src = ''.join(traceback.format_list(self.source_traceback)) msg += 'Future/Task created at (most recent call last):\n' msg += '%s\n' % src.rstrip() msg += ''.join(self.tb).rstrip() self.loop.call_exception_handler({'message': msg}) class Future: """This class is *almost* compatible with concurrent.futures.Future. Differences: - This class is not thread-safe. - result() and exception() do not take a timeout argument and raise an exception when the future isn't done yet. - Callbacks registered with add_done_callback() are always called via the event loop's call_soon(). - This class is not compatible with the wait() and as_completed() methods in the concurrent.futures package. (In Python 3.4 or later we may be able to unify the implementations.) """ # Class variables serving as defaults for instance variables. _state = _PENDING _result = None _exception = None _loop = None _source_traceback = None # This field is used for a dual purpose: # - Its presence is a marker to declare that a class implements # the Future protocol (i.e. is intended to be duck-type compatible). # The value must also be not-None, to enable a subclass to declare # that it is not compatible by setting this to None. # - It is set by __iter__() below so that Task._step() can tell # the difference between `yield from Future()` (correct) vs. # `yield Future()` (incorrect). _asyncio_future_blocking = False _log_traceback = False def __init__(self, *, loop=None): """Initialize the future. The optional event_loop argument allows explicitly setting the event loop object used by the future. If it's not provided, the future uses the default event loop. """ if loop is None: self._loop = events.get_event_loop() else: self._loop = loop self._callbacks = [] if self._loop.get_debug(): self._source_traceback = events.extract_stack(sys._getframe(1)) _repr_info = base_futures._future_repr_info def __repr__(self): return '<%s %s>' % (self.__class__.__name__, ' '.join(self._repr_info())) # On Python 3.3 and older, objects with a destructor part of a reference # cycle are never destroyed. It's not more the case on Python 3.4 thanks # to the PEP 442. if compat.PY34: def __del__(self): if not self._log_traceback: # set_exception() was not called, or result() or exception() # has consumed the exception return exc = self._exception context = { 'message': ('%s exception was never retrieved' % self.__class__.__name__), 'exception': exc, 'future': self, } if self._source_traceback: context['source_traceback'] = self._source_traceback self._loop.call_exception_handler(context) def cancel(self): """Cancel the future and schedule callbacks. If the future is already done or cancelled, return False. Otherwise, change the future's state to cancelled, schedule the callbacks and return True. """ self._log_traceback = False if self._state != _PENDING: return False self._state = _CANCELLED self._schedule_callbacks() return True def _schedule_callbacks(self): """Internal: Ask the event loop to call all callbacks. The callbacks are scheduled to be called as soon as possible. Also clears the callback list. """ callbacks = self._callbacks[:] if not callbacks: return self._callbacks[:] = [] for callback in callbacks: self._loop.call_soon(callback, self) def cancelled(self): """Return True if the future was cancelled.""" return self._state == _CANCELLED # Don't implement running(); see http://bugs.python.org/issue18699 def done(self): """Return True if the future is done. Done means either that a result / exception are available, or that the future was cancelled. """ return self._state != _PENDING def result(self): """Return the result this future represents. If the future has been cancelled, raises CancelledError. If the future's result isn't yet available, raises InvalidStateError. If the future is done and has an exception set, this exception is raised. """ if self._state == _CANCELLED: raise CancelledError if self._state != _FINISHED: raise InvalidStateError('Result is not ready.') self._log_traceback = False if self._exception is not None: raise self._exception return self._result def exception(self): """Return the exception that was set on this future. The exception (or None if no exception was set) is returned only if the future is done. If the future has been cancelled, raises CancelledError. If the future isn't done yet, raises InvalidStateError. """ if self._state == _CANCELLED: raise CancelledError if self._state != _FINISHED: raise InvalidStateError('Exception is not set.') self._log_traceback = False return self._exception def add_done_callback(self, fn): """Add a callback to be run when the future becomes done. The callback is called with a single argument - the future object. If the future is already done when this is called, the callback is scheduled with call_soon. """ if self._state != _PENDING: self._loop.call_soon(fn, self) else: self._callbacks.append(fn) # New method not in PEP 3148. def remove_done_callback(self, fn): """Remove all instances of a callback from the "call when done" list. Returns the number of callbacks removed. """ filtered_callbacks = [f for f in self._callbacks if f != fn] removed_count = len(self._callbacks) - len(filtered_callbacks) if removed_count: self._callbacks[:] = filtered_callbacks return removed_count # So-called internal methods (note: no set_running_or_notify_cancel()). def set_result(self, result): """Mark the future done and set its result. If the future is already done when this method is called, raises InvalidStateError. """ if self._state != _PENDING: raise InvalidStateError('{}: {!r}'.format(self._state, self)) self._result = result self._state = _FINISHED self._schedule_callbacks() def set_exception(self, exception): """Mark the future done and set an exception. If the future is already done when this method is called, raises InvalidStateError. """ if self._state != _PENDING: raise InvalidStateError('{}: {!r}'.format(self._state, self)) if isinstance(exception, type): exception = exception() if type(exception) is StopIteration: raise TypeError("StopIteration interacts badly with generators " "and cannot be raised into a Future") self._exception = exception self._state = _FINISHED self._schedule_callbacks() if compat.PY34: self._log_traceback = True else: self._tb_logger = _TracebackLogger(self, exception) # Arrange for the logger to be activated after all callbacks # have had a chance to call result() or exception(). self._loop.call_soon(self._tb_logger.activate) def __iter__(self): if not self.done(): self._asyncio_future_blocking = True yield self # This tells Task to wait for completion. assert self.done(), "yield from wasn't used with future" return self.result() # May raise too. if compat.PY35: __await__ = __iter__ # make compatible with 'await' expression # Needed for testing purposes. _PyFuture = Future def _set_result_unless_cancelled(fut, result): """Helper setting the result only if the future was not cancelled.""" if fut.cancelled(): return fut.set_result(result) def _set_concurrent_future_state(concurrent, source): """Copy state from a future to a concurrent.futures.Future.""" assert source.done() if source.cancelled(): concurrent.cancel() if not concurrent.set_running_or_notify_cancel(): return exception = source.exception() if exception is not None: concurrent.set_exception(exception) else: result = source.result() concurrent.set_result(result) def _copy_future_state(source, dest): """Internal helper to copy state from another Future. The other Future may be a concurrent.futures.Future. """ assert source.done() if dest.cancelled(): return assert not dest.done() if source.cancelled(): dest.cancel() else: exception = source.exception() if exception is not None: dest.set_exception(exception) else: result = source.result() dest.set_result(result) def _chain_future(source, destination): """Chain two futures so that when one completes, so does the other. The result (or exception) of source will be copied to destination. If destination is cancelled, source gets cancelled too. Compatible with both asyncio.Future and concurrent.futures.Future. """ if not isfuture(source) and not isinstance(source, concurrent.futures.Future): raise TypeError('A future is required for source argument') if not isfuture(destination) and not isinstance(destination, concurrent.futures.Future): raise TypeError('A future is required for destination argument') source_loop = source._loop if isfuture(source) else None dest_loop = destination._loop if isfuture(destination) else None def _set_state(future, other): if isfuture(future): _copy_future_state(other, future) else: _set_concurrent_future_state(future, other) def _call_check_cancel(destination): if destination.cancelled(): if source_loop is None or source_loop is dest_loop: source.cancel() else: source_loop.call_soon_threadsafe(source.cancel) def _call_set_state(source): if (destination.cancelled() and dest_loop is not None and dest_loop.is_closed()): return if dest_loop is None or dest_loop is source_loop: _set_state(destination, source) else: dest_loop.call_soon_threadsafe(_set_state, destination, source) destination.add_done_callback(_call_check_cancel) source.add_done_callback(_call_set_state) def wrap_future(future, *, loop=None): """Wrap concurrent.futures.Future object.""" if isfuture(future): return future assert isinstance(future, concurrent.futures.Future), \ 'concurrent.futures.Future is expected, got {!r}'.format(future) if loop is None: loop = events.get_event_loop() new_future = loop.create_future() _chain_future(future, new_future) return new_future try: import _asyncio except ImportError: pass else: # _CFuture is needed for tests. Future = _CFuture = _asyncio.Future
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