"""A Future class similar to the one in PEP 3148.""" __all__ = ( 'Future', 'wrap_future', 'isfuture', ) import concurrent.futures import contextvars import logging import sys from . import base_futures from . import events from . import exceptions from . import format_helpers 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 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 _cancel_message = None # A saved CancelledError for later chaining as an exception context. _cancelled_exc = 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 # `await Future()` or`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 = format_helpers.extract_stack( sys._getframe(1)) _repr_info = base_futures._future_repr_info def __repr__(self): return '<{} {}>'.format(self.__class__.__name__, ' '.join(self._repr_info())) 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': f'{self.__class__.__name__} exception was never retrieved', 'exception': exc, 'future': self, } if self._source_traceback: context['source_traceback'] = self._source_traceback self._loop.call_exception_handler(context) def __class_getitem__(cls, type): return cls @property def _log_traceback(self): return self.__log_traceback @_log_traceback.setter def _log_traceback(self, val): if bool(val): raise ValueError('_log_traceback can only be set to False') self.__log_traceback = False def get_loop(self): """Return the event loop the Future is bound to.""" loop = self._loop if loop is None: raise RuntimeError("Future object is not initialized.") return loop def _make_cancelled_error(self): """Create the CancelledError to raise if the Future is cancelled. This should only be called once when handling a cancellation since it erases the saved context exception value. """ if self._cancel_message is None: exc = exceptions.CancelledError() else: exc = exceptions.CancelledError(self._cancel_message) exc.__context__ = self._cancelled_exc # Remove the reference since we don't need this anymore. self._cancelled_exc = None return exc def cancel(self, msg=None): """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._cancel_message = msg 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, ctx in callbacks: self._loop.call_soon(callback, self, context=ctx) 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: exc = self._make_cancelled_error() raise exc if self._state != _FINISHED: raise exceptions.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: exc = self._make_cancelled_error() raise exc if self._state != _FINISHED: raise exceptions.InvalidStateError('Exception is not set.') self.__log_traceback = False return self._exception def add_done_callback(self, fn, *, context=None): """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, context=context) else: if context is None: context = contextvars.copy_context() self._callbacks.append((fn, context)) # 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, ctx) for (f, ctx) 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 exceptions.InvalidStateError(f'{self._state}: {self!r}') 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 exceptions.InvalidStateError(f'{self._state}: {self!r}') 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() self.__log_traceback = True def __await__(self): if not self.done(): self._asyncio_future_blocking = True yield self # This tells Task to wait for completion. if not self.done(): raise RuntimeError("await wasn't used with future") return self.result() # May raise too. __iter__ = __await__ # make compatible with 'yield from'. # Needed for testing purposes. _PyFuture = Future def _get_loop(fut): # Tries to call Future.get_loop() if it's available. # Otherwise fallbacks to using the old '_loop' property. try: get_loop = fut.get_loop except AttributeError: pass else: return get_loop() return fut._loop 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 _convert_future_exc(exc): exc_class = type(exc) if exc_class is concurrent.futures.CancelledError: return exceptions.CancelledError(*exc.args) elif exc_class is concurrent.futures.TimeoutError: return exceptions.TimeoutError(*exc.args) elif exc_class is concurrent.futures.InvalidStateError: return exceptions.InvalidStateError(*exc.args) else: return exc 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(_convert_future_exc(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(_convert_future_exc(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 = _get_loop(source) if isfuture(source) else None dest_loop = _get_loop(destination) 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), \ f'concurrent.futures.Future is expected, got {future!r}' 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