text: """Support for tasks, coroutines and the scheduler."""
  
  __all__ = ['Task',
             'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
             'wait', 'wait_for', 'as_completed', 'sleep', 'async',
             'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe',
             ]
  
  import concurrent.futures
  import functools
  import inspect
  import warnings
  import weakref
  
  from . import base_tasks
  from . import compat
  from . import coroutines
  from . import events
  from . import futures
  from .coroutines import coroutine
  
  
  class Task(futures.Future):
      """A coroutine wrapped in a Future."""
  
      # An important invariant maintained while a Task not done:
      #
      # - Either _fut_waiter is None, and _step() is scheduled;
      # - or _fut_waiter is some Future, and _step() is *not* scheduled.
      #
      # The only transition from the latter to the former is through
      # _wakeup().  When _fut_waiter is not None, one of its callbacks
      # must be _wakeup().
  
      # Weak set containing all tasks alive.
      _all_tasks = weakref.WeakSet()
  
      # Dictionary containing tasks that are currently active in
      # all running event loops.  {EventLoop: Task}
      _current_tasks = {}
  
      # If False, don't log a message if the task is destroyed whereas its
      # status is still pending
      _log_destroy_pending = True
  
      @classmethod
      def current_task(cls, loop=None):
          """Return the currently running task in an event loop or None.
  
          By default the current task for the current event loop is returned.
  
          None is returned when called not in the context of a Task.
          """
          if loop is None:
              loop = events.get_event_loop()
          return cls._current_tasks.get(loop)
  
      @classmethod
      def all_tasks(cls, loop=None):
          """Return a set of all tasks for an event loop.
  
          By default all tasks for the current event loop are returned.
          """
          if loop is None:
              loop = events.get_event_loop()
          return {t for t in cls._all_tasks if t._loop is loop}
  
      def __init__(self, coro, *, loop=None):
          assert coroutines.iscoroutine(coro), repr(coro)
          super().__init__(loop=loop)
          if self._source_traceback:
              del self._source_traceback[-1]
          self._coro = coro
          self._fut_waiter = None
          self._must_cancel = False
          self._loop.call_soon(self._step)
          self.__class__._all_tasks.add(self)
  
      def __del__(self):
          if self._state == futures._PENDING and self._log_destroy_pending:
              context = {
                  'task': self,
                  'message': 'Task was destroyed but it is pending!',
              }
              if self._source_traceback:
                  context['source_traceback'] = self._source_traceback
              self._loop.call_exception_handler(context)
          futures.Future.__del__(self)
  
      def _repr_info(self):
          return base_tasks._task_repr_info(self)
  
      def get_stack(self, *, limit=None):
          """Return the list of stack frames for this task's coroutine.
  
          If the coroutine is not done, this returns the stack where it is
          suspended.  If the coroutine has completed successfully or was
          cancelled, this returns an empty list.  If the coroutine was
          terminated by an exception, this returns the list of traceback
          frames.
  
          The frames are always ordered from oldest to newest.
  
          The optional limit gives the maximum number of frames to
          return; by default all available frames are returned.  Its
          meaning differs depending on whether a stack or a traceback is
          returned: the newest frames of a stack are returned, but the
          oldest frames of a traceback are returned.  (This matches the
          behavior of the traceback module.)
  
          For reasons beyond our control, only one stack frame is
          returned for a suspended coroutine.
          """
          return base_tasks._task_get_stack(self, limit)
  
      def print_stack(self, *, limit=None, file=None):
          """Print the stack or traceback for this task's coroutine.
  
          This produces output similar to that of the traceback module,
          for the frames retrieved by get_stack().  The limit argument
          is passed to get_stack().  The file argument is an I/O stream
          to which the output is written; by default output is written
          to sys.stderr.
          """
          return base_tasks._task_print_stack(self, limit, file)
  
      def cancel(self):
          """Request that this task cancel itself.
  
          This arranges for a CancelledError to be thrown into the
          wrapped coroutine on the next cycle through the event loop.
          The coroutine then has a chance to clean up or even deny
          the request using try/except/finally.
  
          Unlike Future.cancel, this does not guarantee that the
          task will be cancelled: the exception might be caught and
          acted upon, delaying cancellation of the task or preventing
          cancellation completely.  The task may also return a value or
          raise a different exception.
  
          Immediately after this method is called, Task.cancelled() will
          not return True (unless the task was already cancelled).  A
          task will be marked as cancelled when the wrapped coroutine
          terminates with a CancelledError exception (even if cancel()
          was not called).
          """
          self._log_traceback = False
          if self.done():
              return False
          if self._fut_waiter is not None:
              if self._fut_waiter.cancel():
                  # Leave self._fut_waiter; it may be a Task that
                  # catches and ignores the cancellation so we may have
                  # to cancel it again later.
                  return True
          # It must be the case that self._step is already scheduled.
          self._must_cancel = True
          return True
  
      def _step(self, exc=None):
          assert not self.done(), \
              '_step(): already done: {!r}, {!r}'.format(self, exc)
          if self._must_cancel:
              if not isinstance(exc, futures.CancelledError):
                  exc = futures.CancelledError()
              self._must_cancel = False
          coro = self._coro
          self._fut_waiter = None
  
          self.__class__._current_tasks[self._loop] = self
          # Call either coro.throw(exc) or coro.send(None).
          try:
              if exc is None:
                  # We use the `send` method directly, because coroutines
                  # don't have `__iter__` and `__next__` methods.
                  result = coro.send(None)
              else:
                  result = coro.throw(exc)
          except StopIteration as exc:
              if self._must_cancel:
                  # Task is cancelled right before coro stops.
                  self._must_cancel = False
                  self.set_exception(futures.CancelledError())
              else:
                  self.set_result(exc.value)
          except futures.CancelledError:
              super().cancel()  # I.e., Future.cancel(self).
          except Exception as exc:
              self.set_exception(exc)
          except BaseException as exc:
              self.set_exception(exc)
              raise
          else:
              blocking = getattr(result, '_asyncio_future_blocking', None)
              if blocking is not None:
                  # Yielded Future must come from Future.__iter__().
                  if result._loop is not self._loop:
                      self._loop.call_soon(
                          self._step,
                          RuntimeError(
                              'Task {!r} got Future {!r} attached to a '
                              'different loop'.format(self, result)))
                  elif blocking:
                      if result is self:
                          self._loop.call_soon(
                              self._step,
                              RuntimeError(
                                  'Task cannot await on itself: {!r}'.format(
                                      self)))
                      else:
                          result._asyncio_future_blocking = False
                          result.add_done_callback(self._wakeup)
                          self._fut_waiter = result
                          if self._must_cancel:
                              if self._fut_waiter.cancel():
                                  self._must_cancel = False
                  else:
                      self._loop.call_soon(
                          self._step,
                          RuntimeError(
                              'yield was used instead of yield from '
                              'in task {!r} with {!r}'.format(self, result)))
              elif result is None:
                  # Bare yield relinquishes control for one event loop iteration.
                  self._loop.call_soon(self._step)
              elif inspect.isgenerator(result):
                  # Yielding a generator is just wrong.
                  self._loop.call_soon(
                      self._step,
                      RuntimeError(
                          'yield was used instead of yield from for '
                          'generator in task {!r} with {}'.format(
                              self, result)))
              else:
                  # Yielding something else is an error.
                  self._loop.call_soon(
                      self._step,
                      RuntimeError(
                          'Task got bad yield: {!r}'.format(result)))
          finally:
              self.__class__._current_tasks.pop(self._loop)
              self = None  # Needed to break cycles when an exception occurs.
  
      def _wakeup(self, future):
          try:
              future.result()
          except Exception as exc:
              # This may also be a cancellation.
              self._step(exc)
          else:
              # Don't pass the value of `future.result()` explicitly,
              # as `Future.__iter__` and `Future.__await__` don't need it.
              # If we call `_step(value, None)` instead of `_step()`,
              # Python eval loop would use `.send(value)` method call,
              # instead of `__next__()`, which is slower for futures
              # that return non-generator iterators from their `__iter__`.
              self._step()
          self = None  # Needed to break cycles when an exception occurs.
  
  
  _PyTask = Task
  
  
  try:
      import _asyncio
  except ImportError:
      pass
  else:
      # _CTask is needed for tests.
      Task = _CTask = _asyncio.Task
  
  
  # wait() and as_completed() similar to those in PEP 3148.
  
  FIRST_COMPLETED = concurrent.futures.FIRST_COMPLETED
  FIRST_EXCEPTION = concurrent.futures.FIRST_EXCEPTION
  ALL_COMPLETED = concurrent.futures.ALL_COMPLETED
  
  
  @coroutine
  def wait(fs, *, loop=None, timeout=None, return_when=ALL_COMPLETED):
      """Wait for the Futures and coroutines given by fs to complete.
  
      The sequence futures must not be empty.
  
      Coroutines will be wrapped in Tasks.
  
      Returns two sets of Future: (done, pending).
  
      Usage:
  
          done, pending = yield from asyncio.wait(fs)
  
      Note: This does not raise TimeoutError! Futures that aren't done
      when the timeout occurs are returned in the second set.
      """
      if futures.isfuture(fs) or coroutines.iscoroutine(fs):
          raise TypeError("expect a list of futures, not %s" % type(fs).__name__)
      if not fs:
          raise ValueError('Set of coroutines/Futures is empty.')
      if return_when not in (FIRST_COMPLETED, FIRST_EXCEPTION, ALL_COMPLETED):
          raise ValueError('Invalid return_when value: {}'.format(return_when))
  
      if loop is None:
          loop = events.get_event_loop()
  
      fs = {ensure_future(f, loop=loop) for f in set(fs)}
  
      return (yield from _wait(fs, timeout, return_when, loop))
  
  
  def _release_waiter(waiter, *args):
      if not waiter.done():
          waiter.set_result(None)
  
  
  @coroutine
  def wait_for(fut, timeout, *, loop=None):
      """Wait for the single Future or coroutine to complete, with timeout.
  
      Coroutine will be wrapped in Task.
  
      Returns result of the Future or coroutine.  When a timeout occurs,
      it cancels the task and raises TimeoutError.  To avoid the task
      cancellation, wrap it in shield().
  
      If the wait is cancelled, the task is also cancelled.
  
      This function is a coroutine.
      """
      if loop is None:
          loop = events.get_event_loop()
  
      if timeout is None:
          return (yield from fut)
  
      waiter = loop.create_future()
      timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
      cb = functools.partial(_release_waiter, waiter)
  
      fut = ensure_future(fut, loop=loop)
      fut.add_done_callback(cb)
  
      try:
          # wait until the future completes or the timeout
          try:
              yield from waiter
          except futures.CancelledError:
              fut.remove_done_callback(cb)
              fut.cancel()
              raise
  
          if fut.done():
              return fut.result()
          else:
              fut.remove_done_callback(cb)
              fut.cancel()
              raise futures.TimeoutError()
      finally:
          timeout_handle.cancel()
  
  
  @coroutine
  def _wait(fs, timeout, return_when, loop):
      """Internal helper for wait() and wait_for().
  
      The fs argument must be a collection of Futures.
      """
      assert fs, 'Set of Futures is empty.'
      waiter = loop.create_future()
      timeout_handle = None
      if timeout is not None:
          timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
      counter = len(fs)
  
      def _on_completion(f):
          nonlocal counter
          counter -= 1
          if (counter <= 0 or
              return_when == FIRST_COMPLETED or
              return_when == FIRST_EXCEPTION and (not f.cancelled() and
                                                  f.exception() is not None)):
              if timeout_handle is not None:
                  timeout_handle.cancel()
              if not waiter.done():
                  waiter.set_result(None)
  
      for f in fs:
          f.add_done_callback(_on_completion)
  
      try:
          yield from waiter
      finally:
          if timeout_handle is not None:
              timeout_handle.cancel()
  
      done, pending = set(), set()
      for f in fs:
          f.remove_done_callback(_on_completion)
          if f.done():
              done.add(f)
          else:
              pending.add(f)
      return done, pending
  
  
  # This is *not* a @coroutine!  It is just an iterator (yielding Futures).
  def as_completed(fs, *, loop=None, timeout=None):
      """Return an iterator whose values are coroutines.
  
      When waiting for the yielded coroutines you'll get the results (or
      exceptions!) of the original Futures (or coroutines), in the order
      in which and as soon as they complete.
  
      This differs from PEP 3148; the proper way to use this is:
  
          for f in as_completed(fs):
              result = yield from f  # The 'yield from' may raise.
              # Use result.
  
      If a timeout is specified, the 'yield from' will raise
      TimeoutError when the timeout occurs before all Futures are done.
  
      Note: The futures 'f' are not necessarily members of fs.
      """
      if futures.isfuture(fs) or coroutines.iscoroutine(fs):
          raise TypeError("expect a list of futures, not %s" % type(fs).__name__)
      loop = loop if loop is not None else events.get_event_loop()
      todo = {ensure_future(f, loop=loop) for f in set(fs)}
      from .queues import Queue  # Import here to avoid circular import problem.
      done = Queue(loop=loop)
      timeout_handle = None
  
      def _on_timeout():
          for f in todo:
              f.remove_done_callback(_on_completion)
              done.put_nowait(None)  # Queue a dummy value for _wait_for_one().
          todo.clear()  # Can't do todo.remove(f) in the loop.
  
      def _on_completion(f):
          if not todo:
              return  # _on_timeout() was here first.
          todo.remove(f)
          done.put_nowait(f)
          if not todo and timeout_handle is not None:
              timeout_handle.cancel()
  
      @coroutine
      def _wait_for_one():
          f = yield from done.get()
          if f is None:
              # Dummy value from _on_timeout().
              raise futures.TimeoutError
          return f.result()  # May raise f.exception().
  
      for f in todo:
          f.add_done_callback(_on_completion)
      if todo and timeout is not None:
          timeout_handle = loop.call_later(timeout, _on_timeout)
      for _ in range(len(todo)):
          yield _wait_for_one()
  
  
  @coroutine
  def sleep(delay, result=None, *, loop=None):
      """Coroutine that completes after a given time (in seconds)."""
      if delay == 0:
          yield
          return result
  
      if loop is None:
          loop = events.get_event_loop()
      future = loop.create_future()
      h = future._loop.call_later(delay,
                                  futures._set_result_unless_cancelled,
                                  future, result)
      try:
          return (yield from future)
      finally:
          h.cancel()
  
  
  def async_(coro_or_future, *, loop=None):
      """Wrap a coroutine in a future.
  
      If the argument is a Future, it is returned directly.
  
      This function is deprecated in 3.5. Use asyncio.ensure_future() instead.
      """
  
      warnings.warn("asyncio.async() function is deprecated, use ensure_future()",
                    DeprecationWarning,
                    stacklevel=2)
  
      return ensure_future(coro_or_future, loop=loop)
  
  # Silence DeprecationWarning:
  globals()['async'] = async_
  async_.__name__ = 'async'
  del async_
  
  
  def ensure_future(coro_or_future, *, loop=None):
      """Wrap a coroutine or an awaitable in a future.
  
      If the argument is a Future, it is returned directly.
      """
      if futures.isfuture(coro_or_future):
          if loop is not None and loop is not coro_or_future._loop:
              raise ValueError('loop argument must agree with Future')
          return coro_or_future
      elif coroutines.iscoroutine(coro_or_future):
          if loop is None:
              loop = events.get_event_loop()
          task = loop.create_task(coro_or_future)
          if task._source_traceback:
              del task._source_traceback[-1]
          return task
      elif compat.PY35 and inspect.isawaitable(coro_or_future):
          return ensure_future(_wrap_awaitable(coro_or_future), loop=loop)
      else:
          raise TypeError('An asyncio.Future, a coroutine or an awaitable is '
                          'required')
  
  
  @coroutine
  def _wrap_awaitable(awaitable):
      """Helper for asyncio.ensure_future().
  
      Wraps awaitable (an object with __await__) into a coroutine
      that will later be wrapped in a Task by ensure_future().
      """
      return (yield from awaitable.__await__())
  
  
  class _GatheringFuture(futures.Future):
      """Helper for gather().
  
      This overrides cancel() to cancel all the children and act more
      like Task.cancel(), which doesn't immediately mark itself as
      cancelled.
      """
  
      def __init__(self, children, *, loop=None):
          super().__init__(loop=loop)
          self._children = children
  
      def cancel(self):
          if self.done():
              return False
          ret = False
          for child in self._children:
              if child.cancel():
                  ret = True
          return ret
  
  
  def gather(*coros_or_futures, loop=None, return_exceptions=False):
      """Return a future aggregating results from the given coroutines
      or futures.
  
      Coroutines will be wrapped in a future and scheduled in the event
      loop. They will not necessarily be scheduled in the same order as
      passed in.
  
      All futures must share the same event loop.  If all the tasks are
      done successfully, the returned future's result is the list of
      results (in the order of the original sequence, not necessarily
      the order of results arrival).  If *return_exceptions* is True,
      exceptions in the tasks are treated the same as successful
      results, and gathered in the result list; otherwise, the first
      raised exception will be immediately propagated to the returned
      future.
  
      Cancellation: if the outer Future is cancelled, all children (that
      have not completed yet) are also cancelled.  If any child is
      cancelled, this is treated as if it raised CancelledError --
      the outer Future is *not* cancelled in this case.  (This is to
      prevent the cancellation of one child to cause other children to
      be cancelled.)
      """
      if not coros_or_futures:
          if loop is None:
              loop = events.get_event_loop()
          outer = loop.create_future()
          outer.set_result([])
          return outer
  
      arg_to_fut = {}
      for arg in set(coros_or_futures):
          if not futures.isfuture(arg):
              fut = ensure_future(arg, loop=loop)
              if loop is None:
                  loop = fut._loop
              # The caller cannot control this future, the "destroy pending task"
              # warning should not be emitted.
              fut._log_destroy_pending = False
          else:
              fut = arg
              if loop is None:
                  loop = fut._loop
              elif fut._loop is not loop:
                  raise ValueError("futures are tied to different event loops")
          arg_to_fut[arg] = fut
  
      children = [arg_to_fut[arg] for arg in coros_or_futures]
      nchildren = len(children)
      outer = _GatheringFuture(children, loop=loop)
      nfinished = 0
      results = [None] * nchildren
  
      def _done_callback(i, fut):
          nonlocal nfinished
          if outer.done():
              if not fut.cancelled():
                  # Mark exception retrieved.
                  fut.exception()
              return
  
          if fut.cancelled():
              res = futures.CancelledError()
              if not return_exceptions:
                  outer.set_exception(res)
                  return
          elif fut._exception is not None:
              res = fut.exception()  # Mark exception retrieved.
              if not return_exceptions:
                  outer.set_exception(res)
                  return
          else:
              res = fut._result
          results[i] = res
          nfinished += 1
          if nfinished == nchildren:
              outer.set_result(results)
  
      for i, fut in enumerate(children):
          fut.add_done_callback(functools.partial(_done_callback, i))
      return outer
  
  
  def shield(arg, *, loop=None):
      """Wait for a future, shielding it from cancellation.
  
      The statement
  
          res = yield from shield(something())
  
      is exactly equivalent to the statement
  
          res = yield from something()
  
      *except* that if the coroutine containing it is cancelled, the
      task running in something() is not cancelled.  From the POV of
      something(), the cancellation did not happen.  But its caller is
      still cancelled, so the yield-from expression still raises
      CancelledError.  Note: If something() is cancelled by other means
      this will still cancel shield().
  
      If you want to completely ignore cancellation (not recommended)
      you can combine shield() with a try/except clause, as follows:
  
          try:
              res = yield from shield(something())
          except CancelledError:
              res = None
      """
      inner = ensure_future(arg, loop=loop)
      if inner.done():
          # Shortcut.
          return inner
      loop = inner._loop
      outer = loop.create_future()
  
      def _done_callback(inner):
          if outer.cancelled():
              if not inner.cancelled():
                  # Mark inner's result as retrieved.
                  inner.exception()
              return
  
          if inner.cancelled():
              outer.cancel()
          else:
              exc = inner.exception()
              if exc is not None:
                  outer.set_exception(exc)
              else:
                  outer.set_result(inner.result())
  
      inner.add_done_callback(_done_callback)
      return outer
  
  
  def run_coroutine_threadsafe(coro, loop):
      """Submit a coroutine object to a given event loop.
  
      Return a concurrent.futures.Future to access the result.
      """
      if not coroutines.iscoroutine(coro):
          raise TypeError('A coroutine object is required')
      future = concurrent.futures.Future()
  
      def callback():
          try:
              futures._chain_future(ensure_future(coro, loop=loop), future)
          except Exception as exc:
              if future.set_running_or_notify_cancel():
                  future.set_exception(exc)
              raise
  
      loop.call_soon_threadsafe(callback)
      return future
  