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388 lines
12 KiB
388 lines
12 KiB
6 years ago
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"""A Future class similar to the one in PEP 3148."""
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__all__ = (
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'CancelledError', 'TimeoutError', 'InvalidStateError',
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'Future', 'wrap_future', 'isfuture',
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)
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import concurrent.futures
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import contextvars
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import logging
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import sys
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from . import base_futures
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from . import events
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from . import format_helpers
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CancelledError = base_futures.CancelledError
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InvalidStateError = base_futures.InvalidStateError
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TimeoutError = base_futures.TimeoutError
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isfuture = base_futures.isfuture
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_PENDING = base_futures._PENDING
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_CANCELLED = base_futures._CANCELLED
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_FINISHED = base_futures._FINISHED
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STACK_DEBUG = logging.DEBUG - 1 # heavy-duty debugging
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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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- This class is not thread-safe.
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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().
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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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_source_traceback = None
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# This field is used for a dual purpose:
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# - Its presence is a marker to declare that a class implements
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# the Future protocol (i.e. is intended to be duck-type compatible).
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# The value must also be not-None, to enable a subclass to declare
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# that it is not compatible by setting this to None.
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# - It is set by __iter__() below so that Task._step() can tell
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# the difference between
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# `await Future()` or`yield from Future()` (correct) vs.
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# `yield Future()` (incorrect).
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_asyncio_future_blocking = False
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__log_traceback = False
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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 explicitly setting 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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if self._loop.get_debug():
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self._source_traceback = format_helpers.extract_stack(
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sys._getframe(1))
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_repr_info = base_futures._future_repr_info
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def __repr__(self):
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return '<{} {}>'.format(self.__class__.__name__,
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' '.join(self._repr_info()))
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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':
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f'{self.__class__.__name__} exception was never retrieved',
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'exception': exc,
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'future': self,
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}
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if self._source_traceback:
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context['source_traceback'] = self._source_traceback
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self._loop.call_exception_handler(context)
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@property
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def _log_traceback(self):
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return self.__log_traceback
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@_log_traceback.setter
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def _log_traceback(self, val):
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if bool(val):
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raise ValueError('_log_traceback can only be set to False')
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self.__log_traceback = False
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def get_loop(self):
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"""Return the event loop the Future is bound to."""
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return self._loop
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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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self.__log_traceback = False
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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, ctx in callbacks:
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self._loop.call_soon(callback, self, context=ctx)
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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._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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return self._exception
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def add_done_callback(self, fn, *, context=None):
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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, context=context)
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else:
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if context is None:
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context = contextvars.copy_context()
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self._callbacks.append((fn, context))
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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, ctx)
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for (f, ctx) in self._callbacks
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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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if type(exception) is StopIteration:
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raise TypeError("StopIteration interacts badly with generators "
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"and cannot be raised into a Future")
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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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self.__log_traceback = True
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def __await__(self):
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if not self.done():
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self._asyncio_future_blocking = True
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yield self # This tells Task to wait for completion.
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if not self.done():
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raise RuntimeError("await wasn't used with future")
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return self.result() # May raise too.
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__iter__ = __await__ # make compatible with 'yield from'.
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# Needed for testing purposes.
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_PyFuture = Future
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def _get_loop(fut):
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# Tries to call Future.get_loop() if it's available.
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# Otherwise fallbacks to using the old '_loop' property.
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try:
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get_loop = fut.get_loop
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except AttributeError:
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pass
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else:
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return get_loop()
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return fut._loop
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def _set_result_unless_cancelled(fut, result):
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"""Helper setting the result only if the future was not cancelled."""
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if fut.cancelled():
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return
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fut.set_result(result)
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def _set_concurrent_future_state(concurrent, source):
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"""Copy state from a future to a concurrent.futures.Future."""
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assert source.done()
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if source.cancelled():
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concurrent.cancel()
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if not concurrent.set_running_or_notify_cancel():
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return
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exception = source.exception()
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if exception is not None:
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concurrent.set_exception(exception)
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else:
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result = source.result()
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concurrent.set_result(result)
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def _copy_future_state(source, dest):
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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 source.done()
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if dest.cancelled():
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return
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assert not dest.done()
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if source.cancelled():
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dest.cancel()
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else:
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exception = source.exception()
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if exception is not None:
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dest.set_exception(exception)
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else:
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result = source.result()
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dest.set_result(result)
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def _chain_future(source, destination):
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"""Chain two futures so that when one completes, so does the other.
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The result (or exception) of source will be copied to destination.
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If destination is cancelled, source gets cancelled too.
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Compatible with both asyncio.Future and concurrent.futures.Future.
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"""
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if not isfuture(source) and not isinstance(source,
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concurrent.futures.Future):
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raise TypeError('A future is required for source argument')
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if not isfuture(destination) and not isinstance(destination,
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concurrent.futures.Future):
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raise TypeError('A future is required for destination argument')
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source_loop = _get_loop(source) if isfuture(source) else None
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dest_loop = _get_loop(destination) if isfuture(destination) else None
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def _set_state(future, other):
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if isfuture(future):
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_copy_future_state(other, future)
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else:
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_set_concurrent_future_state(future, other)
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def _call_check_cancel(destination):
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if destination.cancelled():
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if source_loop is None or source_loop is dest_loop:
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source.cancel()
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else:
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source_loop.call_soon_threadsafe(source.cancel)
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def _call_set_state(source):
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if (destination.cancelled() and
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dest_loop is not None and dest_loop.is_closed()):
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return
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if dest_loop is None or dest_loop is source_loop:
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_set_state(destination, source)
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else:
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dest_loop.call_soon_threadsafe(_set_state, destination, source)
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destination.add_done_callback(_call_check_cancel)
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source.add_done_callback(_call_set_state)
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def wrap_future(future, *, loop=None):
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"""Wrap concurrent.futures.Future object."""
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if isfuture(future):
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return future
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assert isinstance(future, concurrent.futures.Future), \
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f'concurrent.futures.Future is expected, got {future!r}'
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if loop is None:
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loop = events.get_event_loop()
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new_future = loop.create_future()
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_chain_future(future, new_future)
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return new_future
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try:
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import _asyncio
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except ImportError:
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pass
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else:
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# _CFuture is needed for tests.
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Future = _CFuture = _asyncio.Future
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