# Source code for tornado.gen

    """``tornado4.gen`` is a generator-based interface to make it easier to
    work in an asynchronous environment.  Code using the ``gen`` module
    is technically asynchronous, but it is written as a single generator
    instead of a collection of separate functions.

    For example, the following asynchronous handler:

    .. testcode::

        class AsyncHandler(RequestHandler):
            @asynchronous
            def get(self):
                http_client = AsyncHTTPClient()
                http_client.fetch("http://example.com",
                                  callback=self.on_fetch)

            def on_fetch(self, response):
                do_something_with_response(response)
                self.render("template.html")

    .. testoutput::
       :hide:

    could be written with ``gen`` as:

    .. testcode::

        class GenAsyncHandler(RequestHandler):
            @gen.coroutine
            def get(self):
                http_client = AsyncHTTPClient()
                response = yield http_client.fetch("http://example.com")
                do_something_with_response(response)
                self.render("template.html")

    .. testoutput::
       :hide:

    Most asynchronous functions in Tornado return a `.Future`;
    yielding this object returns its `~.Future.result`.

    You can also yield a list or dict of ``Futures``, which will be
    started at the same time and run in parallel; a list or dict of results will
    be returned when they are all finished:

    .. testcode::

        @gen.coroutine
        def get(self):
            http_client = AsyncHTTPClient()
            response1, response2 = yield [http_client.fetch(url1),
                                          http_client.fetch(url2)]
            response_dict = yield dict(response3=http_client.fetch(url3),
                                       response4=http_client.fetch(url4))
            response3 = response_dict['response3']
            response4 = response_dict['response4']

    .. testoutput::
       :hide:

    If the `~functools.singledispatch` library is available (standard in
    Python 3.4, available via the `singledispatch
    <https://pypi.python.org/pypi/singledispatch>`_ package on older
    versions), additional types of objects may be yielded. Tornado includes
    support for ``asyncio.Future`` and Twisted's ``Deferred`` class when
    ``tornado4.platform.asyncio`` and ``tornado4.platform.twisted`` are imported.
    See the `convert_yielded` function to extend this mechanism.

    .. versionchanged:: 3.2
       Dict support added.

    .. versionchanged:: 4.1
       Support added for yielding ``asyncio`` Futures and Twisted Deferreds
       via ``singledispatch``.

    """
    from __future__ import absolute_import, division, print_function

    import collections
    import functools
    import itertools
    import os
    import sys
    import textwrap
    import types
    import weakref

    from tornado4.concurrent import Future, TracebackFuture, is_future, chain_future
    from tornado4.ioloop import IOLoop
    from tornado4.log import app_log
    from tornado4 import stack_context
    from tornado4.util import PY3, raise_exc_info

    try:
        try:
            # py34+
            from functools import singledispatch  # type: ignore
        except ImportError:
            from singledispatch import singledispatch  # backport
    except ImportError:
        # In most cases, singledispatch is required (to avoid
        # difficult-to-diagnose problems in which the functionality
        # available differs depending on which invisble packages are
        # installed). However, in Google App Engine third-party
        # dependencies are more trouble so we allow this module to be
        # imported without it.
        if 'APPENGINE_RUNTIME' not in os.environ:
            raise
        singledispatch = None

    try:
        try:
            # py35+
            from collections.abc import Generator as GeneratorType  # type: ignore
        except ImportError:
            from backports_abc import Generator as GeneratorType  # type: ignore

        try:
            # py35+
            from inspect import isawaitable  # type: ignore
        except ImportError:
            from backports_abc import isawaitable
    except ImportError:
        if 'APPENGINE_RUNTIME' not in os.environ:
            raise
        from types import GeneratorType

        def isawaitable(x):  # type: ignore
            return False

    if PY3:
        import builtins
    else:
        import __builtin__ as builtins

    class KeyReuseError(Exception):
        pass

    class UnknownKeyError(Exception):
        pass

    class LeakedCallbackError(Exception):
        pass

    class BadYieldError(Exception):
        pass

    class ReturnValueIgnoredError(Exception):
        pass

    class TimeoutError(Exception):
        """Exception raised by ``with_timeout``."""

    def _value_from_stopiteration(e):
        try:
            # StopIteration has a value attribute beginning in py33.
            # So does our Return class.
            return e.value
        except AttributeError:
            pass
        try:
            # Cython backports coroutine functionality by putting the value in
            # e.args[0].
            return e.args[0]
        except (AttributeError, IndexError):
            return None

    def engine(func):
        """Callback-oriented decorator for asynchronous generators.

        This is an older interface; for new code that does not need to be
        compatible with versions of Tornado older than 3.0 the
        `coroutine` decorator is recommended instead.

        This decorator is similar to `coroutine`, except it does not
        return a `.Future` and the ``callback`` argument is not treated
        specially.

        In most cases, functions decorated with `engine` should take
        a ``callback`` argument and invoke it with their result when
        they are finished.  One notable exception is the
        `~tornado4.web.RequestHandler` :ref:`HTTP verb methods <verbs>`,
        which use ``self.finish()`` in place of a callback argument.
        """
        func = _make_coroutine_wrapper(func, replace_callback=False)

        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            future = func(*args, **kwargs)

            def final_callback(future):
                if future.result() is not None:
                    raise ReturnValueIgnoredError(
                        "@gen.engine functions cannot return values: %r" %
                        (future.result(),))
            # The engine interface doesn't give us any way to return
            # errors but to raise them into the stack context.
            # Save the stack context here to use when the Future has resolved.
            future.add_done_callback(stack_context.wrap(final_callback))
        return wrapper

    def coroutine(func, replace_callback=True):
        """Decorator for asynchronous generators.

        Any generator that yields objects from this module must be wrapped
        in either this decorator or `engine`.

        Coroutines may "return" by raising the special exception
        `Return(value) <Return>`.  In Python 3.3+, it is also possible for
        the function to simply use the ``return value`` statement (prior to
        Python 3.3 generators were not allowed to also return values).
        In all versions of Python a coroutine that simply wishes to exit
        early may use the ``return`` statement without a value.

        Functions with this decorator return a `.Future`.  Additionally,
        they may be called with a ``callback`` keyword argument, which
        will be invoked with the future's result when it resolves.  If the
        coroutine fails, the callback will not be run and an exception
        will be raised into the surrounding `.StackContext`.  The
        ``callback`` argument is not visible inside the decorated
        function; it is handled by the decorator itself.

        From the caller's perspective, ``@gen.coroutine`` is similar to
        the combination of ``@return_future`` and ``@gen.engine``.

        .. warning::

           When exceptions occur inside a coroutine, the exception
           information will be stored in the `.Future` object. You must
           examine the result of the `.Future` object, or the exception
           may go unnoticed by your code. This means yielding the function
           if called from another coroutine, using something like
           `.IOLoop.run_sync` for top-level calls, or passing the `.Future`
           to `.IOLoop.add_future`.

        """
        return _make_coroutine_wrapper(func, replace_callback=True)

    # Ties lifetime of runners to their result futures. Github Issue #1769
    # Generators, like any object in Python, must be strong referenced
    # in order to not be cleaned up by the garbage collector. When using
    # coroutines, the Runner object is what strong-refs the inner
    # generator. However, the only item that strong-reffed the Runner
    # was the last Future that the inner generator yielded (via the
    # Future's internal done_callback list). Usually this is enough, but
    # it is also possible for this Future to not have any strong references
    # other than other objects referenced by the Runner object (usually
    # when using other callback patterns and/or weakrefs). In this
    # situation, if a garbage collection ran, a cycle would be detected and
    # Runner objects could be destroyed along with their inner generators
    # and everything in their local scope.
    # This map provides strong references to Runner objects as long as
    # their result future objects also have strong references (typically
    # from the parent coroutine's Runner). This keeps the coroutine's
    # Runner alive.
    _futures_to_runners = weakref.WeakKeyDictionary()

    def _make_coroutine_wrapper(func, replace_callback):
        """The inner workings of ``@gen.coroutine`` and ``@gen.engine``.

        The two decorators differ in their treatment of the ``callback``
        argument, so we cannot simply implement ``@engine`` in terms of
        ``@coroutine``.
        """
        # On Python 3.5, set the coroutine flag on our generator, to allow it
        # to be used with 'await'.
        wrapped = func
        if hasattr(types, 'coroutine'):
            func = types.coroutine(func)

        @functools.wraps(wrapped)
        def wrapper(*args, **kwargs):
            future = TracebackFuture()

            if replace_callback and 'callback' in kwargs:
                callback = kwargs.pop('callback')
                IOLoop.current().add_future(
                    future, lambda future: callback(future.result()))

            try:
                result = func(*args, **kwargs)
            except (Return, StopIteration) as e:
                result = _value_from_stopiteration(e)
            except Exception:
                future.set_exc_info(sys.exc_info())
                return future
            else:
                if isinstance(result, GeneratorType):
                    # Inline the first iteration of Runner.run.  This lets us
                    # avoid the cost of creating a Runner when the coroutine
                    # never actually yields, which in turn allows us to
                    # use "optional" coroutines in critical path code without
                    # performance penalty for the synchronous case.
                    try:
                        orig_stack_contexts = stack_context._state.contexts
                        yielded = next(result)
                        if stack_context._state.contexts is not orig_stack_contexts:
                            yielded = TracebackFuture()
                            yielded.set_exception(
                                stack_context.StackContextInconsistentError(
                                    'stack_context inconsistency (probably caused '
                                    'by yield within a "with StackContext" block)'))
                    except (StopIteration, Return) as e:
                        future.set_result(_value_from_stopiteration(e))
                    except Exception:
                        future.set_exc_info(sys.exc_info())
                    else:
                        _futures_to_runners[future] = Runner(result, future, yielded)
                    yielded = None
                    try:
                        return future
                    finally:
                        # Subtle memory optimization: if next() raised an exception,
                        # the future's exc_info contains a traceback which
                        # includes this stack frame.  This creates a cycle,
                        # which will be collected at the next full GC but has
                        # been shown to greatly increase memory usage of
                        # benchmarks (relative to the refcount-based scheme
                        # used in the absence of cycles).  We can avoid the
                        # cycle by clearing the local variable after we return it.
                        future = None
            future.set_result(result)
            return future

        wrapper.__wrapped__ = wrapped
        wrapper.__tornado_coroutine__ = True
        return wrapper

    def is_coroutine_function(func):
        """Return whether *func* is a coroutine function, i.e. a function
        wrapped with `~.gen.coroutine`.

        .. versionadded:: 4.5
        """
        return getattr(func, '__tornado_coroutine__', False)

    class Return(Exception):
        """Special exception to return a value from a `coroutine`.

        If this exception is raised, its value argument is used as the
        result of the coroutine::

            @gen.coroutine
            def fetch_json(url):
                response = yield AsyncHTTPClient().fetch(url)
                raise gen.Return(json_decode(response.body))

        In Python 3.3, this exception is no longer necessary: the ``return``
        statement can be used directly to return a value (previously
        ``yield`` and ``return`` with a value could not be combined in the
        same function).

        By analogy with the return statement, the value argument is optional,
        but it is never necessary to ``raise gen.Return()``.  The ``return``
        statement can be used with no arguments instead.
        """
        def __init__(self, value=None):
            super(Return, self).__init__()
            self.value = value
            # Cython recognizes subclasses of StopIteration with a .args tuple.
            self.args = (value,)

    class WaitIterator(object):
        """Provides an iterator to yield the results of futures as they finish.

        Yielding a set of futures like this:

        ``results = yield [future1, future2]``

        pauses the coroutine until both ``future1`` and ``future2``
        return, and then restarts the coroutine with the results of both
        futures. If either future is an exception, the expression will
        raise that exception and all the results will be lost.

        If you need to get the result of each future as soon as possible,
        or if you need the result of some futures even if others produce
        errors, you can use ``WaitIterator``::

          wait_iterator = gen.WaitIterator(future1, future2)
          while not wait_iterator.done():
              try:
                  result = yield wait_iterator.next()
              except Exception as e:
                  print("Error {} from {}".format(e, wait_iterator.current_future))
              else:
                  print("Result {} received from {} at {}".format(
                      result, wait_iterator.current_future,
                      wait_iterator.current_index))

        Because results are returned as soon as they are available the
        output from the iterator *will not be in the same order as the
        input arguments*. If you need to know which future produced the
        current result, you can use the attributes
        ``WaitIterator.current_future``, or ``WaitIterator.current_index``
        to get the index of the future from the input list. (if keyword
        arguments were used in the construction of the `WaitIterator`,
        ``current_index`` will use the corresponding keyword).

        On Python 3.5, `WaitIterator` implements the async iterator
        protocol, so it can be used with the ``async for`` statement (note
        that in this version the entire iteration is aborted if any value
        raises an exception, while the previous example can continue past
        individual errors)::

          async for result in gen.WaitIterator(future1, future2):
              print("Result {} received from {} at {}".format(
                  result, wait_iterator.current_future,
                  wait_iterator.current_index))

        .. versionadded:: 4.1

        .. versionchanged:: 4.3
           Added ``async for`` support in Python 3.5.

        """
        def __init__(self, *args, **kwargs):
            if args and kwargs:
                raise ValueError(
                    "You must provide args or kwargs, not both")

            if kwargs:
                self._unfinished = dict((f, k) for (k, f) in kwargs.items())
                futures = list(kwargs.values())
            else:
                self._unfinished = dict((f, i) for (i, f) in enumerate(args))
                futures = args

            self._finished = collections.deque()
            self.current_index = self.current_future = None
            self._running_future = None

            for future in futures:
                future.add_done_callback(self._done_callback)

        def done(self):
            """Returns True if this iterator has no more results."""
            if self._finished or self._unfinished:
                return False
            # Clear the 'current' values when iteration is done.
            self.current_index = self.current_future = None
            return True

        def next(self):
            """Returns a `.Future` that will yield the next available result.

            Note that this `.Future` will not be the same object as any of
            the inputs.
            """
            self._running_future = TracebackFuture()

            if self._finished:
                self._return_result(self._finished.popleft())

            return self._running_future

        def _done_callback(self, done):
            if self._running_future and not self._running_future.done():
                self._return_result(done)
            else:
                self._finished.append(done)

        def _return_result(self, done):
            """Called set the returned future's state that of the future
            we yielded, and set the current future for the iterator.
            """
            chain_future(done, self._running_future)

            self.current_future = done
            self.current_index = self._unfinished.pop(done)

        def __aiter__(self):
            return self

        def __anext__(self):
            if self.done():
                # Lookup by name to silence pyflakes on older versions.
                raise getattr(builtins, 'StopAsyncIteration')()
            return self.next()

    class YieldPoint(object):
        """Base class for objects that may be yielded from the generator.

        .. deprecated:: 4.0
           Use `Futures <.Future>` instead.
        """
        def start(self, runner):
            """Called by the runner after the generator has yielded.

            No other methods will be called on this object before ``start``.
            """
            raise NotImplementedError()

        def is_ready(self):
            """Called by the runner to determine whether to resume the generator.

            Returns a boolean; may be called more than once.
            """
            raise NotImplementedError()

        def get_result(self):
            """Returns the value to use as the result of the yield expression.

            This method will only be called once, and only after `is_ready`
            has returned true.
            """
            raise NotImplementedError()

    class Callback(YieldPoint):
        """Returns a callable object that will allow a matching `Wait` to proceed.

        The key may be any value suitable for use as a dictionary key, and is
        used to match ``Callbacks`` to their corresponding ``Waits``.  The key
        must be unique among outstanding callbacks within a single run of the
        generator function, but may be reused across different runs of the same
        function (so constants generally work fine).

        The callback may be called with zero or one arguments; if an argument
        is given it will be returned by `Wait`.

        .. deprecated:: 4.0
           Use `Futures <.Future>` instead.
        """
        def __init__(self, key):
            self.key = key

        def start(self, runner):
            self.runner = runner
            runner.register_callback(self.key)

        def is_ready(self):
            return True

        def get_result(self):
            return self.runner.result_callback(self.key)

    class Wait(YieldPoint):
        """Returns the argument passed to the result of a previous `Callback`.

        .. deprecated:: 4.0
           Use `Futures <.Future>` instead.
        """
        def __init__(self, key):
            self.key = key

        def start(self, runner):
            self.runner = runner

        def is_ready(self):
            return self.runner.is_ready(self.key)

        def get_result(self):
            return self.runner.pop_result(self.key)

    class WaitAll(YieldPoint):
        """Returns the results of multiple previous `Callbacks <Callback>`.

        The argument is a sequence of `Callback` keys, and the result is
        a list of results in the same order.

        `WaitAll` is equivalent to yielding a list of `Wait` objects.

        .. deprecated:: 4.0
           Use `Futures <.Future>` instead.
        """
        def __init__(self, keys):
            self.keys = keys

        def start(self, runner):
            self.runner = runner

        def is_ready(self):
            return all(self.runner.is_ready(key) for key in self.keys)

        def get_result(self):
            return [self.runner.pop_result(key) for key in self.keys]

    def Task(func, *args, **kwargs):
        """Adapts a callback-based asynchronous function for use in coroutines.

        Takes a function (and optional additional arguments) and runs it with
        those arguments plus a ``callback`` keyword argument.  The argument passed
        to the callback is returned as the result of the yield expression.

        .. versionchanged:: 4.0
           ``gen.Task`` is now a function that returns a `.Future`, instead of
           a subclass of `YieldPoint`.  It still behaves the same way when
           yielded.
        """
        future = Future()

        def handle_exception(typ, value, tb):
            if future.done():
                return False
            future.set_exc_info((typ, value, tb))
            return True

        def set_result(result):
            if future.done():
                return
            future.set_result(result)
        with stack_context.ExceptionStackContext(handle_exception):
            func(*args, callback=_argument_adapter(set_result), **kwargs)
        return future

    class YieldFuture(YieldPoint):
        def __init__(self, future, io_loop=None):
            """Adapts a `.Future` to the `YieldPoint` interface.

            .. versionchanged:: 4.1
               The ``io_loop`` argument is deprecated.
            """
            self.future = future
            self.io_loop = io_loop or IOLoop.current()

        def start(self, runner):
            if not self.future.done():
                self.runner = runner
                self.key = object()
                runner.register_callback(self.key)
                self.io_loop.add_future(self.future, runner.result_callback(self.key))
            else:
                self.runner = None
                self.result_fn = self.future.result

        def is_ready(self):
            if self.runner is not None:
                return self.runner.is_ready(self.key)
            else:
                return True

        def get_result(self):
            if self.runner is not None:
                return self.runner.pop_result(self.key).result()
            else:
                return self.result_fn()

    def _contains_yieldpoint(children):
        """Returns True if ``children`` contains any YieldPoints.

        ``children`` may be a dict or a list, as used by `MultiYieldPoint`
        and `multi_future`.
        """
        if isinstance(children, dict):
            return any(isinstance(i, YieldPoint) for i in children.values())
        if isinstance(children, list):
            return any(isinstance(i, YieldPoint) for i in children)
        return False

    def multi(children, quiet_exceptions=()):
        """Runs multiple asynchronous operations in parallel.

        ``children`` may either be a list or a dict whose values are
        yieldable objects. ``multi()`` returns a new yieldable
        object that resolves to a parallel structure containing their
        results. If ``children`` is a list, the result is a list of
        results in the same order; if it is a dict, the result is a dict
        with the same keys.

        That is, ``results = yield multi(list_of_futures)`` is equivalent
        to::

            results = []
            for future in list_of_futures:
                results.append(yield future)

        If any children raise exceptions, ``multi()`` will raise the first
        one. All others will be logged, unless they are of types
        contained in the ``quiet_exceptions`` argument.

        If any of the inputs are `YieldPoints <YieldPoint>`, the returned
        yieldable object is a `YieldPoint`. Otherwise, returns a `.Future`.
        This means that the result of `multi` can be used in a native
        coroutine if and only if all of its children can be.

        In a ``yield``-based coroutine, it is not normally necessary to
        call this function directly, since the coroutine runner will
        do it automatically when a list or dict is yielded. However,
        it is necessary in ``await``-based coroutines, or to pass
        the ``quiet_exceptions`` argument.

        This function is available under the names ``multi()`` and ``Multi()``
        for historical reasons.

        .. versionchanged:: 4.2
           If multiple yieldables fail, any exceptions after the first
           (which is raised) will be logged. Added the ``quiet_exceptions``
           argument to suppress this logging for selected exception types.

        .. versionchanged:: 4.3
           Replaced the class ``Multi`` and the function ``multi_future``
           with a unified function ``multi``. Added support for yieldables
           other than `YieldPoint` and `.Future`.

        """
        if _contains_yieldpoint(children):
            return MultiYieldPoint(children, quiet_exceptions=quiet_exceptions)
        else:
            return multi_future(children, quiet_exceptions=quiet_exceptions)

    Multi = multi

    class MultiYieldPoint(YieldPoint):
        """Runs multiple asynchronous operations in parallel.

        This class is similar to `multi`, but it always creates a stack
        context even when no children require it. It is not compatible with
        native coroutines.

        .. versionchanged:: 4.2
           If multiple ``YieldPoints`` fail, any exceptions after the first
           (which is raised) will be logged. Added the ``quiet_exceptions``
           argument to suppress this logging for selected exception types.

        .. versionchanged:: 4.3
           Renamed from ``Multi`` to ``MultiYieldPoint``. The name ``Multi``
           remains as an alias for the equivalent `multi` function.

        .. deprecated:: 4.3
           Use `multi` instead.
        """
        def __init__(self, children, quiet_exceptions=()):
            self.keys = None
            if isinstance(children, dict):
                self.keys = list(children.keys())
                children = children.values()
            self.children = []
            for i in children:
                if not isinstance(i, YieldPoint):
                    i = convert_yielded(i)
                if is_future(i):
                    i = YieldFuture(i)
                self.children.append(i)
            assert all(isinstance(i, YieldPoint) for i in self.children)
            self.unfinished_children = set(self.children)
            self.quiet_exceptions = quiet_exceptions

        def start(self, runner):
            for i in self.children:
                i.start(runner)

        def is_ready(self):
            finished = list(itertools.takewhile(
                lambda i: i.is_ready(), self.unfinished_children))
            self.unfinished_children.difference_update(finished)
            return not self.unfinished_children

        def get_result(self):
            result_list = []
            exc_info = None
            for f in self.children:
                try:
                    result_list.append(f.get_result())
                except Exception as e:
                    if exc_info is None:
                        exc_info = sys.exc_info()
                    else:
                        if not isinstance(e, self.quiet_exceptions):
                            app_log.error("Multiple exceptions in yield list",
                                          exc_info=True)
            if exc_info is not None:
                raise_exc_info(exc_info)
            if self.keys is not None:
                return dict(zip(self.keys, result_list))
            else:
                return list(result_list)

    def multi_future(children, quiet_exceptions=()):
        """Wait for multiple asynchronous futures in parallel.

        This function is similar to `multi`, but does not support
        `YieldPoints <YieldPoint>`.

        .. versionadded:: 4.0

        .. versionchanged:: 4.2
           If multiple ``Futures`` fail, any exceptions after the first (which is
           raised) will be logged. Added the ``quiet_exceptions``
           argument to suppress this logging for selected exception types.

        .. deprecated:: 4.3
           Use `multi` instead.
        """
        if isinstance(children, dict):
            keys = list(children.keys())
            children = children.values()
        else:
            keys = None
        children = list(map(convert_yielded, children))
        assert all(is_future(i) for i in children)
        unfinished_children = set(children)

        future = Future()
        if not children:
            future.set_result({} if keys is not None else [])

        def callback(f):
            unfinished_children.remove(f)
            if not unfinished_children:
                result_list = []
                for f in children:
                    try:
                        result_list.append(f.result())
                    except Exception as e:
                        if future.done():
                            if not isinstance(e, quiet_exceptions):
                                app_log.error("Multiple exceptions in yield list",
                                              exc_info=True)
                        else:
                            future.set_exc_info(sys.exc_info())
                if not future.done():
                    if keys is not None:
                        future.set_result(dict(zip(keys, result_list)))
                    else:
                        future.set_result(result_list)

        listening = set()
        for f in children:
            if f not in listening:
                listening.add(f)
                f.add_done_callback(callback)
        return future

    def maybe_future(x):
        """Converts ``x`` into a `.Future`.

        If ``x`` is already a `.Future`, it is simply returned; otherwise
        it is wrapped in a new `.Future`.  This is suitable for use as
        ``result = yield gen.maybe_future(f())`` when you don't know whether
        ``f()`` returns a `.Future` or not.

        .. deprecated:: 4.3
           This function only handles ``Futures``, not other yieldable objects.
           Instead of `maybe_future`, check for the non-future result types
           you expect (often just ``None``), and ``yield`` anything unknown.
        """
        if is_future(x):
            return x
        else:
            fut = Future()
            fut.set_result(x)
            return fut

    def with_timeout(timeout, future, io_loop=None, quiet_exceptions=()):
        """Wraps a `.Future` (or other yieldable object) in a timeout.

        Raises `TimeoutError` if the input future does not complete before
        ``timeout``, which may be specified in any form allowed by
        `.IOLoop.add_timeout` (i.e. a `datetime.timedelta` or an absolute time
        relative to `.IOLoop.time`)

        If the wrapped `.Future` fails after it has timed out, the exception
        will be logged unless it is of a type contained in ``quiet_exceptions``
        (which may be an exception type or a sequence of types).

        Does not support `YieldPoint` subclasses.

        .. versionadded:: 4.0

        .. versionchanged:: 4.1
           Added the ``quiet_exceptions`` argument and the logging of unhandled
           exceptions.

        .. versionchanged:: 4.4
           Added support for yieldable objects other than `.Future`.
        """
        # TODO: allow YieldPoints in addition to other yieldables?
        # Tricky to do with stack_context semantics.
        #
        # It's tempting to optimize this by cancelling the input future on timeout
        # instead of creating a new one, but A) we can't know if we are the only
        # one waiting on the input future, so cancelling it might disrupt other
        # callers and B) concurrent futures can only be cancelled while they are
        # in the queue, so cancellation cannot reliably bound our waiting time.
        future = convert_yielded(future)
        result = Future()
        chain_future(future, result)
        if io_loop is None:
            io_loop = IOLoop.current()

        def error_callback(future):
            try:
                future.result()
            except Exception as e:
                if not isinstance(e, quiet_exceptions):
                    app_log.error("Exception in Future %r after timeout",
                                  future, exc_info=True)

        def timeout_callback():
            result.set_exception(TimeoutError("Timeout"))
            # In case the wrapped future goes on to fail, log it.
            future.add_done_callback(error_callback)
        timeout_handle = io_loop.add_timeout(
            timeout, timeout_callback)
        if isinstance(future, Future):
            # We know this future will resolve on the IOLoop, so we don't
            # need the extra thread-safety of IOLoop.add_future (and we also
            # don't care about StackContext here.
            future.add_done_callback(
                lambda future: io_loop.remove_timeout(timeout_handle))
        else:
            # concurrent.futures.Futures may resolve on any thread, so we
            # need to route them back to the IOLoop.
            io_loop.add_future(
                future, lambda future: io_loop.remove_timeout(timeout_handle))
        return result

    def sleep(duration):
        """Return a `.Future` that resolves after the given number of seconds.

        When used with ``yield`` in a coroutine, this is a non-blocking
        analogue to `time.sleep` (which should not be used in coroutines
        because it is blocking)::

            yield gen.sleep(0.5)

        Note that calling this function on its own does nothing; you must
        wait on the `.Future` it returns (usually by yielding it).

        .. versionadded:: 4.1
        """
        f = Future()
        IOLoop.current().call_later(duration, lambda: f.set_result(None))
        return f

    _null_future = Future()
    _null_future.set_result(None)

    moment = Future()
    moment.__doc__ = \
        """A special object which may be yielded to allow the IOLoop to run for
    one iteration.

    This is not needed in normal use but it can be helpful in long-running
    coroutines that are likely to yield Futures that are ready instantly.

    Usage: ``yield gen.moment``

    .. versionadded:: 4.0

    .. deprecated:: 4.5
       ``yield None`` is now equivalent to ``yield gen.moment``.
    """
    moment.set_result(None)

    class Runner(object):
        """Internal implementation of `tornado4.gen.engine`.

        Maintains information about pending callbacks and their results.

        The results of the generator are stored in ``result_future`` (a
        `.TracebackFuture`)
        """
        def __init__(self, gen, result_future, first_yielded):
            self.gen = gen
            self.result_future = result_future
            self.future = _null_future
            self.yield_point = None
            self.pending_callbacks = None
            self.results = None
            self.running = False
            self.finished = False
            self.had_exception = False
            self.io_loop = IOLoop.current()
            # For efficiency, we do not create a stack context until we
            # reach a YieldPoint (stack contexts are required for the historical
            # semantics of YieldPoints, but not for Futures).  When we have
            # done so, this field will be set and must be called at the end
            # of the coroutine.
            self.stack_context_deactivate = None
            if self.handle_yield(first_yielded):
                gen = result_future = first_yielded = None
                self.run()

        def register_callback(self, key):
            """Adds ``key`` to the list of callbacks."""
            if self.pending_callbacks is None:
                # Lazily initialize the old-style YieldPoint data structures.
                self.pending_callbacks = set()
                self.results = {}
            if key in self.pending_callbacks:
                raise KeyReuseError("key %r is already pending" % (key,))
            self.pending_callbacks.add(key)

        def is_ready(self, key):
            """Returns true if a result is available for ``key``."""
            if self.pending_callbacks is None or key not in self.pending_callbacks:
                raise UnknownKeyError("key %r is not pending" % (key,))
            return key in self.results

        def set_result(self, key, result):
            """Sets the result for ``key`` and attempts to resume the generator."""
            self.results[key] = result
            if self.yield_point is not None and self.yield_point.is_ready():
                try:
                    self.future.set_result(self.yield_point.get_result())
                except:
                    self.future.set_exc_info(sys.exc_info())
                self.yield_point = None
                self.run()

        def pop_result(self, key):
            """Returns the result for ``key`` and unregisters it."""
            self.pending_callbacks.remove(key)
            return self.results.pop(key)

        def run(self):
            """Starts or resumes the generator, running until it reaches a
            yield point that is not ready.
            """
            if self.running or self.finished:
                return
            try:
                self.running = True
                while True:
                    future = self.future
                    if not future.done():
                        return
                    self.future = None
                    try:
                        orig_stack_contexts = stack_context._state.contexts
                        exc_info = None

                        try:
                            value = future.result()
                        except Exception:
                            self.had_exception = True
                            exc_info = sys.exc_info()
                        future = None

                        if exc_info is not None:
                            try:
                                yielded = self.gen.throw(*exc_info)
                            finally:
                                # Break up a reference to itself
                                # for faster GC on CPython.
                                exc_info = None
                        else:
                            yielded = self.gen.send(value)

                        if stack_context._state.contexts is not orig_stack_contexts:
                            self.gen.throw(
                                stack_context.StackContextInconsistentError(
                                    'stack_context inconsistency (probably caused '
                                    'by yield within a "with StackContext" block)'))
                    except (StopIteration, Return) as e:
                        self.finished = True
                        self.future = _null_future
                        if self.pending_callbacks and not self.had_exception:
                            # If we ran cleanly without waiting on all callbacks
                            # raise an error (really more of a warning).  If we
                            # had an exception then some callbacks may have been
                            # orphaned, so skip the check in that case.
                            raise LeakedCallbackError(
                                "finished without waiting for callbacks %r" %
                                self.pending_callbacks)
                        self.result_future.set_result(_value_from_stopiteration(e))
                        self.result_future = None
                        self._deactivate_stack_context()
                        return
                    except Exception:
                        self.finished = True
                        self.future = _null_future
                        self.result_future.set_exc_info(sys.exc_info())
                        self.result_future = None
                        self._deactivate_stack_context()
                        return
                    if not self.handle_yield(yielded):
                        return
                    yielded = None
            finally:
                self.running = False

        def handle_yield(self, yielded):
            # Lists containing YieldPoints require stack contexts;
            # other lists are handled in convert_yielded.
            if _contains_yieldpoint(yielded):
                yielded = multi(yielded)

            if isinstance(yielded, YieldPoint):
                # YieldPoints are too closely coupled to the Runner to go
                # through the generic convert_yielded mechanism.
                self.future = TracebackFuture()

                def start_yield_point():
                    try:
                        yielded.start(self)
                        if yielded.is_ready():
                            self.future.set_result(
                                yielded.get_result())
                        else:
                            self.yield_point = yielded
                    except Exception:
                        self.future = TracebackFuture()
                        self.future.set_exc_info(sys.exc_info())

                if self.stack_context_deactivate is None:
                    # Start a stack context if this is the first
                    # YieldPoint we've seen.
                    with stack_context.ExceptionStackContext(
                            self.handle_exception) as deactivate:
                        self.stack_context_deactivate = deactivate

                        def cb():
                            start_yield_point()
                            self.run()
                        self.io_loop.add_callback(cb)
                        return False
                else:
                    start_yield_point()
            else:
                try:
                    self.future = convert_yielded(yielded)
                except BadYieldError:
                    self.future = TracebackFuture()
                    self.future.set_exc_info(sys.exc_info())

            if not self.future.done() or self.future is moment:
                def inner(f):
                    # Break a reference cycle to speed GC.
                    f = None # noqa
                    self.run()
                self.io_loop.add_future(
                    self.future, inner)
                return False
            return True

        def result_callback(self, key):
            return stack_context.wrap(_argument_adapter(
                functools.partial(self.set_result, key)))

        def handle_exception(self, typ, value, tb):
            if not self.running and not self.finished:
                self.future = TracebackFuture()
                self.future.set_exc_info((typ, value, tb))
                self.run()
                return True
            else:
                return False

        def _deactivate_stack_context(self):
            if self.stack_context_deactivate is not None:
                self.stack_context_deactivate()
                self.stack_context_deactivate = None

    Arguments = collections.namedtuple('Arguments', ['args', 'kwargs'])

    def _argument_adapter(callback):
        """Returns a function that when invoked runs ``callback`` with one arg.

        If the function returned by this function is called with exactly
        one argument, that argument is passed to ``callback``.  Otherwise
        the args tuple and kwargs dict are wrapped in an `Arguments` object.
        """
        def wrapper(*args, **kwargs):
            if kwargs or len(args) > 1:
                callback(Arguments(args, kwargs))
            elif args:
                callback(args[0])
            else:
                callback(None)
        return wrapper

    # Convert Awaitables into Futures. It is unfortunately possible
    # to have infinite recursion here if those Awaitables assume that
    # we're using a different coroutine runner and yield objects
    # we don't understand. If that happens, the solution is to
    # register that runner's yieldable objects with convert_yielded.
    if sys.version_info >= (3, 3):
        exec(textwrap.dedent("""
        @coroutine
        def _wrap_awaitable(x):
            if hasattr(x, '__await__'):
                x = x.__await__()
            return (yield from x)
        """))
    else:
        # Py2-compatible version for use with Cython.
        # Copied from PEP 380.
        @coroutine
        def _wrap_awaitable(x):
            if hasattr(x, '__await__'):
                _i = x.__await__()
            else:
                _i = iter(x)
            try:
                _y = next(_i)
            except StopIteration as _e:
                _r = _value_from_stopiteration(_e)
            else:
                while 1:
                    try:
                        _s = yield _y
                    except GeneratorExit as _e:
                        try:
                            _m = _i.close
                        except AttributeError:
                            pass
                        else:
                            _m()
                        raise _e
                    except BaseException as _e:
                        _x = sys.exc_info()
                        try:
                            _m = _i.throw
                        except AttributeError:
                            raise _e
                        else:
                            try:
                                _y = _m(*_x)
                            except StopIteration as _e:
                                _r = _value_from_stopiteration(_e)
                                break
                    else:
                        try:
                            if _s is None:
                                _y = next(_i)
                            else:
                                _y = _i.send(_s)
                        except StopIteration as _e:
                            _r = _value_from_stopiteration(_e)
                            break
            raise Return(_r)

    def convert_yielded(yielded):
        """Convert a yielded object into a `.Future`.

        The default implementation accepts lists, dictionaries, and Futures.

        If the `~functools.singledispatch` library is available, this function
        may be extended to support additional types. For example::

            @convert_yielded.register(asyncio.Future)
            def _(asyncio_future):
                return tornado4.platform.asyncio.to_tornado_future(asyncio_future)

        .. versionadded:: 4.1
        """
        # Lists and dicts containing YieldPoints were handled earlier.
        if yielded is None:
            return moment
        elif isinstance(yielded, (list, dict)):
            return multi(yielded)
        elif is_future(yielded):
            return yielded
        elif isawaitable(yielded):
            return _wrap_awaitable(yielded)
        else:
            raise BadYieldError("yielded unknown object %r" % (yielded,))

    if singledispatch is not None:
        convert_yielded = singledispatch(convert_yielded)

        try:
            # If we can import t.p.asyncio, do it for its side effect
            # (registering asyncio.Future with convert_yielded).
            # It's ugly to do this here, but it prevents a cryptic
            # infinite recursion in _wrap_awaitable.
            # Note that even with this, asyncio integration is unlikely
            # to work unless the application also configures AsyncIOLoop,
            # but at least the error messages in that case are more
            # comprehensible than a stack overflow.
            import tornado4.platform.asyncio
        except ImportError:
            pass