"""A Future class similar to the one in PEP 3148."""
__all__ = ['CancelledError', 'TimeoutError',
'InvalidStateError',
'Future', 'wrap_future',
]
import concurrent.futures._base
import logging
import reprlib
import sys
import traceback
from . import compat
from . import events
# States for Future.
_PENDING = 'PENDING'
_CANCELLED = 'CANCELLED'
_FINISHED = 'FINISHED'
Error = concurrent.futures._base.Error
CancelledError = concurrent.futures.CancelledError
TimeoutError = concurrent.futures.TimeoutError
STACK_DEBUG = logging.DEBUG - 1 # heavy-duty debugging
class InvalidStateError(Error):
"""The operation is not allowed in this state."""
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})
[docs]class Future:
"""This class is *almost* compatible with concurrent.futures.Future.
Differences:
- 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_threadsafe().
- 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
_blocking = False # proper use of future (yield vs yield from)
_log_traceback = False # Used for Python 3.4 and later
_tb_logger = None # Used for Python 3.3 only
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 = traceback.extract_stack(sys._getframe(1))
def __format_callbacks(self):
cb = self._callbacks
size = len(cb)
if not size:
cb = ''
def format_cb(callback):
return events._format_callback_source(callback, ())
if size == 1:
cb = format_cb(cb[0])
elif size == 2:
cb = '{}, {}'.format(format_cb(cb[0]), format_cb(cb[1]))
elif size > 2:
cb = '{}, <{} more>, {}'.format(format_cb(cb[0]),
size-2,
format_cb(cb[-1]))
return 'cb=[%s]' % cb
def _repr_info(self):
info = [self._state.lower()]
if self._state == _FINISHED:
if self._exception is not None:
info.append('exception={!r}'.format(self._exception))
else:
# use reprlib to limit the length of the output, especially
# for very long strings
result = reprlib.repr(self._result)
info.append('result={}'.format(result))
if self._callbacks:
info.append(self.__format_callbacks())
if self._source_traceback:
frame = self._source_traceback[-1]
info.append('created at %s:%s' % (frame[0], frame[1]))
return info
def __repr__(self):
info = self._repr_info()
return '<%s %s>' % (self.__class__.__name__, ' '.join(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.
"""
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._tb_logger is not None:
self._tb_logger.clear()
self._tb_logger = None
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
if self._tb_logger is not None:
self._tb_logger.clear()
self._tb_logger = None
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._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
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 isinstance(source, (Future, concurrent.futures.Future)):
raise TypeError('A future is required for source argument')
if not isinstance(destination, (Future, concurrent.futures.Future)):
raise TypeError('A future is required for destination argument')
source_loop = source._loop if isinstance(source, Future) else None
dest_loop = destination._loop if isinstance(destination, Future) else None
def _set_state(future, other):
if isinstance(future, 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 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 isinstance(future, 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