text: """Support for tasks, coroutines and the scheduler."""
  
  __all__ = ['coroutine', 'Task',
             'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
             'wait', 'wait_for', 'as_completed', 'sleep', 'async',
             'gather', 'shield',
             ]
  
  import collections
  import concurrent.futures
  import functools
  import inspect
  import linecache
  import traceback
  import weakref
  
  from . import events
  from . import futures
  from .log import logger
  
  # If you set _DEBUG to true, @coroutine will wrap the resulting
  # generator objects in a CoroWrapper instance (defined below).  That
  # instance will log a message when the generator is never iterated
  # over, which may happen when you forget to use "yield from" with a
  # coroutine call.  Note that the value of the _DEBUG flag is taken
  # when the decorator is used, so to be of any use it must be set
  # before you define your coroutines.  A downside of using this feature
  # is that tracebacks show entries for the CoroWrapper.__next__ method
  # when _DEBUG is true.
  _DEBUG = False
  
  
  class CoroWrapper:
      """Wrapper for coroutine in _DEBUG mode."""
  
      __slot__ = ['gen', 'func']
  
      def __init__(self, gen, func):
          assert inspect.isgenerator(gen), gen
          self.gen = gen
          self.func = func
  
      def __iter__(self):
          return self
  
      def __next__(self):
          return next(self.gen)
  
      def send(self, value):
          return self.gen.send(value)
  
      def throw(self, exc):
          return self.gen.throw(exc)
  
      def close(self):
          return self.gen.close()
  
      def __del__(self):
          frame = self.gen.gi_frame
          if frame is not None and frame.f_lasti == -1:
              func = self.func
              code = func.__code__
              filename = code.co_filename
              lineno = code.co_firstlineno
              logger.error(
                  'Coroutine %r defined at %s:%s was never yielded from',
                  func.__name__, filename, lineno)
  
  
  def coroutine(func):
      """Decorator to mark coroutines.
  
      If the coroutine is not yielded from before it is destroyed,
      an error message is logged.
      """
      if inspect.isgeneratorfunction(func):
          coro = func
      else:
          @functools.wraps(func)
          def coro(*args, **kw):
              res = func(*args, **kw)
              if isinstance(res, futures.Future) or inspect.isgenerator(res):
                  res = yield from res
              return res
  
      if not _DEBUG:
          wrapper = coro
      else:
          @functools.wraps(func)
          def wrapper(*args, **kwds):
              w = CoroWrapper(coro(*args, **kwds), func)
              w.__name__ = coro.__name__
              w.__doc__ = coro.__doc__
              return w
  
      wrapper._is_coroutine = True  # For iscoroutinefunction().
      return wrapper
  
  
  def iscoroutinefunction(func):
      """Return True if func is a decorated coroutine function."""
      return getattr(func, '_is_coroutine', False)
  
  
  def iscoroutine(obj):
      """Return True if obj is a coroutine object."""
      return isinstance(obj, CoroWrapper) or inspect.isgenerator(obj)
  
  
  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()
  
      @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 iscoroutine(coro), repr(coro)  # Not a coroutine function!
          super().__init__(loop=loop)
          self._coro = iter(coro)  # Use the iterator just in case.
          self._fut_waiter = None
          self._must_cancel = False
          self._loop.call_soon(self._step)
          self.__class__._all_tasks.add(self)
  
      def __repr__(self):
          res = super().__repr__()
          if (self._must_cancel and
              self._state == futures._PENDING and
              '<PENDING' in res):
              res = res.replace('<PENDING', '<CANCELLING', 1)
          i = res.find('<')
          if i < 0:
              i = len(res)
          res = res[:i] + '(<{}>)'.format(self._coro.__name__) + res[i:]
          return res
  
      def get_stack(self, *, limit=None):
          """Return the list of stack frames for this task's coroutine.
  
          If the coroutine is active, 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 nummber 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.
          """
          frames = []
          f = self._coro.gi_frame
          if f is not None:
              while f is not None:
                  if limit is not None:
                      if limit <= 0:
                          break
                      limit -= 1
                  frames.append(f)
                  f = f.f_back
              frames.reverse()
          elif self._exception is not None:
              tb = self._exception.__traceback__
              while tb is not None:
                  if limit is not None:
                      if limit <= 0:
                          break
                      limit -= 1
                  frames.append(tb.tb_frame)
                  tb = tb.tb_next
          return frames
  
      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 goes; by default it goes to sys.stderr.
          """
          extracted_list = []
          checked = set()
          for f in self.get_stack(limit=limit):
              lineno = f.f_lineno
              co = f.f_code
              filename = co.co_filename
              name = co.co_name
              if filename not in checked:
                  checked.add(filename)
                  linecache.checkcache(filename)
              line = linecache.getline(filename, lineno, f.f_globals)
              extracted_list.append((filename, lineno, name, line))
          exc = self._exception
          if not extracted_list:
              print('No stack for %r' % self, file=file)
          elif exc is not None:
              print('Traceback for %r (most recent call last):' % self,
                    file=file)
          else:
              print('Stack for %r (most recent call last):' % self,
                    file=file)
          traceback.print_list(extracted_list, file=file)
          if exc is not None:
              for line in traceback.format_exception_only(exc.__class__, exc):
                  print(line, file=file, end='')
  
      def cancel(self):
          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, value=None, exc=None):
          assert not self.done(), \
              '_step(): already done: {!r}, {!r}, {!r}'.format(self, value, 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
          # Call either coro.throw(exc) or coro.send(value).
          try:
              if exc is not None:
                  result = coro.throw(exc)
              elif value is not None:
                  result = coro.send(value)
              else:
                  result = next(coro)
          except StopIteration as exc:
              self.set_result(exc.value)
          except futures.CancelledError as exc:
              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:
              if isinstance(result, futures.Future):
                  # Yielded Future must come from Future.__iter__().
                  if result._blocking:
                      result._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, None,
                          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, None,
                      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, None,
                      RuntimeError(
                          'Task got bad yield: {!r}'.format(result)))
          self = None
  
      def _wakeup(self, future):
          try:
              value = future.result()
          except Exception as exc:
              # This may also be a cancellation.
              self._step(None, exc)
          else:
              self._step(value, None)
          self = None  # Needed to break cycles when an exception occurs.
  
  
  # 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.
  
      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 not fs:
          raise ValueError('Set of coroutines/Futures is empty.')
  
      if loop is None:
          loop = events.get_event_loop()
  
      fs = set(async(f, loop=loop) for f in fs)
  
      if return_when not in (FIRST_COMPLETED, FIRST_EXCEPTION, ALL_COMPLETED):
          raise ValueError('Invalid return_when value: {}'.format(return_when))
      return (yield from _wait(fs, timeout, return_when, loop))
  
  
  def _release_waiter(waiter, value=True, *args):
      if not waiter.done():
          waiter.set_result(value)
  
  
  @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.  Raises TimeoutError when
      timeout occurs.
  
      Usage:
  
          result = yield from asyncio.wait_for(fut, 10.0)
  
      """
      if loop is None:
          loop = events.get_event_loop()
  
      waiter = futures.Future(loop=loop)
      timeout_handle = loop.call_later(timeout, _release_waiter, waiter, False)
      cb = functools.partial(_release_waiter, waiter, True)
  
      fut = async(fut, loop=loop)
      fut.add_done_callback(cb)
  
      try:
          if (yield from waiter):
              return fut.result()
          else:
              fut.remove_done_callback(cb)
              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 = futures.Future(loop=loop)
      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(False)
  
      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, when waited for, are Futures.
  
      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.
  
      Raises TimeoutError if the timeout occurs before all Futures are
      done.
  
      Note: The futures 'f' are not necessarily members of fs.
      """
      loop = loop if loop is not None else events.get_event_loop()
      deadline = None if timeout is None else loop.time() + timeout
      todo = set(async(f, loop=loop) for f in fs)
      completed = collections.deque()
  
      @coroutine
      def _wait_for_one():
          while not completed:
              timeout = None
              if deadline is not None:
                  timeout = deadline - loop.time()
                  if timeout < 0:
                      raise futures.TimeoutError()
              done, pending = yield from _wait(
                  todo, timeout, FIRST_COMPLETED, loop)
              # Multiple callers might be waiting for the same events
              # and getting the same outcome.  Dedupe by updating todo.
              for f in done:
                  if f in todo:
                      todo.remove(f)
                      completed.append(f)
          f = completed.popleft()
          return f.result()  # May raise.
  
      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)."""
      future = futures.Future(loop=loop)
      h = future._loop.call_later(delay, future.set_result, 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.
      """
      if isinstance(coro_or_future, futures.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 iscoroutine(coro_or_future):
          return Task(coro_or_future, loop=loop)
      else:
          raise TypeError('A Future or coroutine is required')
  
  
  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
          for child in self._children:
              child.cancel()
          return True
  
  
  def gather(*coros_or_futures, loop=None, return_exceptions=False):
      """Return a future aggregating results from the given coroutines
      or futures.
  
      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 *result_exception* 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.)
      """
      children = [async(fut, loop=loop) for fut in coros_or_futures]
      n = len(children)
      if n == 0:
          outer = futures.Future(loop=loop)
          outer.set_result([])
          return outer
      if loop is None:
          loop = children[0]._loop
      for fut in children:
          if fut._loop is not loop:
              raise ValueError("futures are tied to different event loops")
      outer = _GatheringFuture(children, loop=loop)
      nfinished = 0
      results = [None] * n
  
      def _done_callback(i, fut):
          nonlocal nfinished
          if outer._state != futures._PENDING:
              if fut._exception is not None:
                  # Mark exception retrieved.
                  fut.exception()
              return
          if fut._state == futures._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 == n:
              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 = async(arg, loop=loop)
      if inner.done():
          # Shortcut.
          return inner
      loop = inner._loop
      outer = futures.Future(loop=loop)
  
      def _done_callback(inner):
          if outer.cancelled():
              # Mark inner's result as retrieved.
              inner.cancelled() or 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
  