371 lines
13 KiB
Python
371 lines
13 KiB
Python
"""A Future class similar to the one in PEP 3148."""
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__all__ = ['CancelledError', 'TimeoutError',
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'InvalidStateError',
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'Future', 'wrap_future',
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]
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import concurrent.futures._base
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import logging
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import sys
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import traceback
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from . import events
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# States for Future.
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_PENDING = 'PENDING'
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_CANCELLED = 'CANCELLED'
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_FINISHED = 'FINISHED'
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_PY34 = sys.version_info >= (3, 4)
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# TODO: Do we really want to depend on concurrent.futures internals?
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Error = concurrent.futures._base.Error
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CancelledError = concurrent.futures.CancelledError
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TimeoutError = concurrent.futures.TimeoutError
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STACK_DEBUG = logging.DEBUG - 1 # heavy-duty debugging
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class InvalidStateError(Error):
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"""The operation is not allowed in this state."""
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# TODO: Show the future, its state, the method, and the required state.
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class _TracebackLogger:
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"""Helper to log a traceback upon destruction if not cleared.
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This solves a nasty problem with Futures and Tasks that have an
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exception set: if nobody asks for the exception, the exception is
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never logged. This violates the Zen of Python: 'Errors should
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never pass silently. Unless explicitly silenced.'
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However, we don't want to log the exception as soon as
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set_exception() is called: if the calling code is written
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properly, it will get the exception and handle it properly. But
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we *do* want to log it if result() or exception() was never called
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-- otherwise developers waste a lot of time wondering why their
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buggy code fails silently.
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An earlier attempt added a __del__() method to the Future class
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itself, but this backfired because the presence of __del__()
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prevents garbage collection from breaking cycles. A way out of
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this catch-22 is to avoid having a __del__() method on the Future
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class itself, but instead to have a reference to a helper object
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with a __del__() method that logs the traceback, where we ensure
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that the helper object doesn't participate in cycles, and only the
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Future has a reference to it.
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The helper object is added when set_exception() is called. When
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the Future is collected, and the helper is present, the helper
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object is also collected, and its __del__() method will log the
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traceback. When the Future's result() or exception() method is
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called (and a helper object is present), it removes the the helper
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object, after calling its clear() method to prevent it from
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logging.
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One downside is that we do a fair amount of work to extract the
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traceback from the exception, even when it is never logged. It
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would seem cheaper to just store the exception object, but that
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references the traceback, which references stack frames, which may
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reference the Future, which references the _TracebackLogger, and
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then the _TracebackLogger would be included in a cycle, which is
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what we're trying to avoid! As an optimization, we don't
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immediately format the exception; we only do the work when
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activate() is called, which call is delayed until after all the
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Future's callbacks have run. Since usually a Future has at least
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one callback (typically set by 'yield from') and usually that
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callback extracts the callback, thereby removing the need to
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format the exception.
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PS. I don't claim credit for this solution. I first heard of it
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in a discussion about closing files when they are collected.
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"""
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__slots__ = ['exc', 'tb', 'loop']
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def __init__(self, exc, loop):
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self.loop = loop
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self.exc = exc
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self.tb = None
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def activate(self):
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exc = self.exc
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if exc is not None:
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self.exc = None
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self.tb = traceback.format_exception(exc.__class__, exc,
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exc.__traceback__)
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def clear(self):
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self.exc = None
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self.tb = None
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def __del__(self):
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if self.tb:
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msg = 'Future/Task exception was never retrieved:\n{tb}'
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context = {
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'message': msg.format(tb=''.join(self.tb)),
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}
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self.loop.call_exception_handler(context)
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class Future:
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"""This class is *almost* compatible with concurrent.futures.Future.
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Differences:
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- result() and exception() do not take a timeout argument and
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raise an exception when the future isn't done yet.
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- Callbacks registered with add_done_callback() are always called
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via the event loop's call_soon_threadsafe().
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- This class is not compatible with the wait() and as_completed()
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methods in the concurrent.futures package.
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(In Python 3.4 or later we may be able to unify the implementations.)
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"""
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# Class variables serving as defaults for instance variables.
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_state = _PENDING
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_result = None
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_exception = None
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_loop = None
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_blocking = False # proper use of future (yield vs yield from)
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_log_traceback = False # Used for Python 3.4 and later
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_tb_logger = None # Used for Python 3.3 only
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def __init__(self, *, loop=None):
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"""Initialize the future.
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The optional event_loop argument allows to explicitly set the event
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loop object used by the future. If it's not provided, the future uses
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the default event loop.
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"""
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if loop is None:
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self._loop = events.get_event_loop()
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else:
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self._loop = loop
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self._callbacks = []
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def __repr__(self):
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res = self.__class__.__name__
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if self._state == _FINISHED:
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if self._exception is not None:
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res += '<exception={!r}>'.format(self._exception)
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else:
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res += '<result={!r}>'.format(self._result)
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elif self._callbacks:
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size = len(self._callbacks)
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if size > 2:
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res += '<{}, [{}, <{} more>, {}]>'.format(
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self._state, self._callbacks[0],
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size-2, self._callbacks[-1])
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else:
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res += '<{}, {}>'.format(self._state, self._callbacks)
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else:
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res += '<{}>'.format(self._state)
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return res
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if _PY34:
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def __del__(self):
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if not self._log_traceback:
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# set_exception() was not called, or result() or exception()
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# has consumed the exception
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return
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exc = self._exception
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context = {
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'message': 'Future/Task exception was never retrieved',
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'exception': exc,
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'future': self,
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}
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self._loop.call_exception_handler(context)
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def cancel(self):
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"""Cancel the future and schedule callbacks.
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If the future is already done or cancelled, return False. Otherwise,
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change the future's state to cancelled, schedule the callbacks and
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return True.
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"""
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if self._state != _PENDING:
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return False
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self._state = _CANCELLED
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self._schedule_callbacks()
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return True
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def _schedule_callbacks(self):
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"""Internal: Ask the event loop to call all callbacks.
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The callbacks are scheduled to be called as soon as possible. Also
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clears the callback list.
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"""
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callbacks = self._callbacks[:]
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if not callbacks:
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return
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self._callbacks[:] = []
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for callback in callbacks:
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self._loop.call_soon(callback, self)
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def cancelled(self):
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"""Return True if the future was cancelled."""
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return self._state == _CANCELLED
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# Don't implement running(); see http://bugs.python.org/issue18699
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def done(self):
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"""Return True if the future is done.
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Done means either that a result / exception are available, or that the
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future was cancelled.
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"""
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return self._state != _PENDING
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def result(self):
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"""Return the result this future represents.
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If the future has been cancelled, raises CancelledError. If the
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future's result isn't yet available, raises InvalidStateError. If
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the future is done and has an exception set, this exception is raised.
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"""
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if self._state == _CANCELLED:
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raise CancelledError
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if self._state != _FINISHED:
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raise InvalidStateError('Result is not ready.')
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self._log_traceback = False
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if self._tb_logger is not None:
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self._tb_logger.clear()
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self._tb_logger = None
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if self._exception is not None:
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raise self._exception
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return self._result
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def exception(self):
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"""Return the exception that was set on this future.
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The exception (or None if no exception was set) is returned only if
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the future is done. If the future has been cancelled, raises
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CancelledError. If the future isn't done yet, raises
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InvalidStateError.
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"""
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if self._state == _CANCELLED:
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raise CancelledError
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if self._state != _FINISHED:
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raise InvalidStateError('Exception is not set.')
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self._log_traceback = False
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if self._tb_logger is not None:
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self._tb_logger.clear()
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self._tb_logger = None
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return self._exception
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def add_done_callback(self, fn):
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"""Add a callback to be run when the future becomes done.
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The callback is called with a single argument - the future object. If
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the future is already done when this is called, the callback is
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scheduled with call_soon.
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"""
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if self._state != _PENDING:
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self._loop.call_soon(fn, self)
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else:
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self._callbacks.append(fn)
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# New method not in PEP 3148.
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def remove_done_callback(self, fn):
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"""Remove all instances of a callback from the "call when done" list.
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Returns the number of callbacks removed.
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"""
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filtered_callbacks = [f for f in self._callbacks if f != fn]
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removed_count = len(self._callbacks) - len(filtered_callbacks)
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if removed_count:
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self._callbacks[:] = filtered_callbacks
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return removed_count
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# So-called internal methods (note: no set_running_or_notify_cancel()).
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def set_result(self, result):
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"""Mark the future done and set its result.
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If the future is already done when this method is called, raises
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InvalidStateError.
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"""
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if self._state != _PENDING:
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raise InvalidStateError('{}: {!r}'.format(self._state, self))
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self._result = result
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self._state = _FINISHED
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self._schedule_callbacks()
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def set_exception(self, exception):
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"""Mark the future done and set an exception.
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If the future is already done when this method is called, raises
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InvalidStateError.
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"""
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if self._state != _PENDING:
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raise InvalidStateError('{}: {!r}'.format(self._state, self))
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if isinstance(exception, type):
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exception = exception()
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self._exception = exception
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self._state = _FINISHED
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self._schedule_callbacks()
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if _PY34:
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self._log_traceback = True
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else:
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self._tb_logger = _TracebackLogger(exception, self._loop)
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# Arrange for the logger to be activated after all callbacks
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# have had a chance to call result() or exception().
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self._loop.call_soon(self._tb_logger.activate)
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# Truly internal methods.
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def _copy_state(self, other):
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"""Internal helper to copy state from another Future.
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The other Future may be a concurrent.futures.Future.
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"""
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assert other.done()
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if self.cancelled():
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return
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assert not self.done()
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if other.cancelled():
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self.cancel()
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else:
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exception = other.exception()
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if exception is not None:
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self.set_exception(exception)
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else:
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result = other.result()
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self.set_result(result)
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def __iter__(self):
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if not self.done():
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self._blocking = True
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yield self # This tells Task to wait for completion.
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assert self.done(), "yield from wasn't used with future"
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return self.result() # May raise too.
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def wrap_future(fut, *, loop=None):
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"""Wrap concurrent.futures.Future object."""
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if isinstance(fut, Future):
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return fut
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assert isinstance(fut, concurrent.futures.Future), \
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'concurrent.futures.Future is expected, got {!r}'.format(fut)
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if loop is None:
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loop = events.get_event_loop()
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new_future = Future(loop=loop)
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def _check_cancel_other(f):
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if f.cancelled():
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fut.cancel()
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new_future.add_done_callback(_check_cancel_other)
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fut.add_done_callback(
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lambda future: loop.call_soon_threadsafe(
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new_future._copy_state, fut))
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return new_future
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