"""A Future class similar to the one in PEP 3148."""
__all__ = ['CancelledError', 'TimeoutError', 'InvalidStateError',
'Future', 'wrap_future', 'isfuture']
import concurrent.futures
from . import base_futures
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
"""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
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
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.source_traceback = future._source_traceback
self.tb = traceback.format_exception(exc.__class__, exc,
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})
"""This class is *almost* compatible with concurrent.futures.Future.
- 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.
# 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
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
self._loop = events.get_event_loop()
if self._loop.get_debug():
self._source_traceback = events.extract_stack(sys._getframe(1))
_repr_info = base_futures._future_repr_info
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
if not self._log_traceback:
# set_exception() was not called, or result() or exception()
# has consumed the exception
'message': ('%s exception was never retrieved'
% self.__class__.__name__),
if self._source_traceback:
context['source_traceback'] = self._source_traceback
self._loop.call_exception_handler(context)
"""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
self._log_traceback = False
if self._state != _PENDING:
self._schedule_callbacks()
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[:]
for callback in callbacks:
self._loop.call_soon(callback, self)
"""Return True if the future was cancelled."""
return self._state == _CANCELLED
# Don't implement running(); see http://bugs.python.org/issue18699
"""Return True if the future is done.
Done means either that a result / exception are available, or that the
return self._state != _PENDING
"""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:
if self._state != _FINISHED:
raise InvalidStateError('Result is not ready.')
self._log_traceback = False
if self._exception is not None:
"""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
if self._state == _CANCELLED:
if self._state != _FINISHED:
raise InvalidStateError('Exception is not set.')
self._log_traceback = False
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)
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)
self._callbacks[:] = filtered_callbacks
# 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
if self._state != _PENDING:
raise InvalidStateError('{}: {!r}'.format(self._state, self))
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
if self._state != _PENDING:
raise InvalidStateError('{}: {!r}'.format(self._state, self))
if isinstance(exception, type):
if type(exception) is StopIteration:
raise TypeError("StopIteration interacts badly with generators "
"and cannot be raised into a Future")
self._exception = exception
self._schedule_callbacks()
self._log_traceback = True
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)
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.
__await__ = __iter__ # make compatible with 'await' expression
# Needed for testing purposes.
def _set_result_unless_cancelled(fut, result):
"""Helper setting the result only if the future was not cancelled."""
def _set_concurrent_future_state(concurrent, source):
"""Copy state from a future to a concurrent.futures.Future."""
if not concurrent.set_running_or_notify_cancel():
exception = source.exception()
if exception is not None:
concurrent.set_exception(exception)
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.
exception = source.exception()
if exception is not None:
dest.set_exception(exception)
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):
_copy_future_state(other, future)
_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_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()):
if dest_loop is None or dest_loop is source_loop:
_set_state(destination, source)
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."""
assert isinstance(future, concurrent.futures.Future), \
'concurrent.futures.Future is expected, got {!r}'.format(future)
loop = events.get_event_loop()
new_future = loop.create_future()
_chain_future(future, new_future)
# _CFuture is needed for tests.
Future = _CFuture = _asyncio.Future
It is recommended that you Edit text format, this type of Fix handles quite a lot in one request
