# Source code for tornado.gen

    """``tornado.gen`` implements generator-based coroutines.

    .. note::

       The "decorator and generator" approach in this module is a
       precursor to native coroutines (using ``async def`` and ``await``)
       which were introduced in Python 3.5. Applications that do not
       require compatibility with older versions of Python should use
       native coroutines instead. Some parts of this module are still
       useful with native coroutines, notably `multi`, `sleep`,
       `WaitIterator`, and `with_timeout`. Some of these functions have
       counterparts in the `asyncio` module which may be used as well,
       although the two may not necessarily be 100% compatible.

    Coroutines provide an easier way to work in an asynchronous
    environment than chaining callbacks. Code using coroutines is
    technically asynchronous, but it is written as a single generator
    instead of a collection of separate functions.

    For example, here's a coroutine-based handler:

    .. 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")

    Asynchronous functions in Tornado return an ``Awaitable`` or `.Future`;
    yielding this object returns its result.

    You can also yield a list or dict of other yieldable objects, 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']

    If ``tornado.platform.twisted`` is imported, it is also possible to
    yield Twisted's ``Deferred`` objects. 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``.

    """

    import asyncio
    import builtins
    import collections
    from collections.abc import Generator
    import concurrent.futures
    import datetime
    import functools
    from functools import singledispatch
    from inspect import isawaitable
    import sys
    import types

    from tornado.concurrent import (
        Future,
        is_future,
        chain_future,
        future_set_exc_info,
        future_add_done_callback,
        future_set_result_unless_cancelled,
    )
    from tornado.ioloop import IOLoop
    from tornado.log import app_log
    from tornado.util import TimeoutError

    try:
        import contextvars
    except ImportError:
        contextvars = None  # type: ignore

    import typing
    from typing import (
        Mapping,
        Union,
        Any,
        Callable,
        List,
        Type,
        Tuple,
        Awaitable,
        Dict,
        Sequence,
        overload,
    )

    if typing.TYPE_CHECKING:
        from typing import Deque, Optional, Set, Iterable  # noqa: F401

    _T = typing.TypeVar("_T")

    _Yieldable = Union[
        None, Awaitable, List[Awaitable], Dict[Any, Awaitable], concurrent.futures.Future
    ]

    class KeyReuseError(Exception):
        pass

    class UnknownKeyError(Exception):
        pass

    class LeakedCallbackError(Exception):
        pass

    class BadYieldError(Exception):
        pass

    class ReturnValueIgnoredError(Exception):
        pass

    def _value_from_stopiteration(e: Union[StopIteration, "Return"]) -> Any:
        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 _create_future() -> Future:
        future = Future()  # type: Future
        # Fixup asyncio debug info by removing extraneous stack entries
        source_traceback = getattr(future, "_source_traceback", ())
        while source_traceback:
            # Each traceback entry is equivalent to a
            # (filename, self.lineno, self.name, self.line) tuple
            filename = source_traceback[-1][0]
            if filename == __file__:
                del source_traceback[-1]
            else:
                break
        return future

    def _fake_ctx_run(f: Callable[..., _T], *args: Any, **kw: Any) -> _T:
        return f(*args, **kw)

    @overload
    def coroutine(
        func: Callable[..., "Generator[Any, Any, _T]"]
    ) -> Callable[..., "Future[_T]"]: ...

    @overload
    def coroutine(func: Callable[..., _T]) -> Callable[..., "Future[_T]"]: ...

    def coroutine(
        func: Union[Callable[..., "Generator[Any, Any, _T]"], Callable[..., _T]]
    ) -> Callable[..., "Future[_T]"]:
        """Decorator for asynchronous generators.

        For compatibility with older versions of Python, coroutines may
        also "return" by raising the special exception `Return(value)
        <Return>`.

        Functions with this decorator return a `.Future`.

        .. 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`.

        .. versionchanged:: 6.0

           The ``callback`` argument was removed. Use the returned
           awaitable object instead.

        """

        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            # type: (*Any, **Any) -> Future[_T]
            # This function is type-annotated with a comment to work around
            # https://bitbucket.org/pypy/pypy/issues/2868/segfault-with-args-type-annotation-in
            future = _create_future()
            if contextvars is not None:
                ctx_run = contextvars.copy_context().run  # type: Callable
            else:
                ctx_run = _fake_ctx_run
            try:
                result = ctx_run(func, *args, **kwargs)
            except (Return, StopIteration) as e:
                result = _value_from_stopiteration(e)
            except Exception:
                future_set_exc_info(future, sys.exc_info())
                try:
                    return future
                finally:
                    # Avoid circular references
                    future = None  # type: ignore
            else:
                if isinstance(result, Generator):
                    # 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:
                        yielded = ctx_run(next, result)
                    except (StopIteration, Return) as e:
                        future_set_result_unless_cancelled(
                            future, _value_from_stopiteration(e)
                        )
                    except Exception:
                        future_set_exc_info(future, sys.exc_info())
                    else:
                        # Provide 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.
                        # We do this by exploiting the public API
                        # add_done_callback() instead of putting a private
                        # attribute on the Future.
                        # (GitHub issues #1769, #2229).
                        runner = Runner(ctx_run, result, future, yielded)
                        future.add_done_callback(lambda _: runner)
                    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  # type: ignore
            future_set_result_unless_cancelled(future, result)
            return future

        wrapper.__wrapped__ = func  # type: ignore
        wrapper.__tornado_coroutine__ = True  # type: ignore
        return wrapper

    def is_coroutine_function(func: Any) -> bool:
        """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`.

        This exception exists for compatibility with older versions of
        Python (before 3.3). In newer code use the ``return`` statement
        instead.

        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))

        By analogy with the return statement, the value argument is optional.
        """

        def __init__(self, value: Any = None) -> None:
            super().__init__()
            self.value = value
            # Cython recognizes subclasses of StopIteration with a .args tuple.
            self.args = (value,)

    class WaitIterator:
        """Provides an iterator to yield the results of awaitables as they finish.

        Yielding a set of awaitables like this:

        ``results = yield [awaitable1, awaitable2]``

        pauses the coroutine until both ``awaitable1`` and ``awaitable2``
        return, and then restarts the coroutine with the results of both
        awaitables. If either awaitable raises an exception, the
        expression will raise that exception and all the results will be
        lost.

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

          wait_iterator = gen.WaitIterator(awaitable1, awaitable2)
          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 awaitable from the input list. (if keyword
        arguments were used in the construction of the `WaitIterator`,
        ``current_index`` will use the corresponding keyword).

        `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.

        """

        _unfinished = {}  # type: Dict[Future, Union[int, str]]

        def __init__(self, *args: Future, **kwargs: Future) -> None:
            if args and kwargs:
                raise ValueError("You must provide args or kwargs, not both")

            if kwargs:
                self._unfinished = {f: k for (k, f) in kwargs.items()}
                futures = list(kwargs.values())  # type: Sequence[Future]
            else:
                self._unfinished = {f: i for (i, f) in enumerate(args)}
                futures = args

            self._finished = collections.deque()  # type: Deque[Future]
            self.current_index = None  # type: Optional[Union[str, int]]
            self.current_future = None  # type: Optional[Future]
            self._running_future = None  # type: Optional[Future]

            for future in futures:
                future_add_done_callback(future, self._done_callback)

        def done(self) -> bool:
            """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) -> Future:
            """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 = Future()

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

            return self._running_future

        def _done_callback(self, done: Future) -> None:
            if self._running_future and not self._running_future.done():
                self._return_result(done)
            else:
                self._finished.append(done)

        def _return_result(self, done: Future) -> Future:
            """Called set the returned future's state that of the future
            we yielded, and set the current future for the iterator.
            """
            if self._running_future is None:
                raise Exception("no future is running")
            chain_future(done, self._running_future)

            res = self._running_future
            self._running_future = None
            self.current_future = done
            self.current_index = self._unfinished.pop(done)

            return res

        def __aiter__(self) -> typing.AsyncIterator:
            return self

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

    @overload
    def multi(
        children: Sequence[_Yieldable],
        quiet_exceptions: Union[Type[Exception], Tuple[Type[Exception], ...]] = (),
    ) -> Future[List]: ...

    @overload
    def multi(
        children: Mapping[Any, _Yieldable],
        quiet_exceptions: Union[Type[Exception], Tuple[Type[Exception], ...]] = (),
    ) -> Future[Dict]: ...

    def multi(
        children: Union[Sequence[_Yieldable], Mapping[Any, _Yieldable]],
        quiet_exceptions: "Union[Type[Exception], Tuple[Type[Exception], ...]]" = (),
    ) -> "Union[Future[List], Future[Dict]]":
        """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.

        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.

        Cancelling a `.Future` returned by ``multi()`` does not cancel its
        children. `asyncio.gather` is similar to ``multi()``, but it does
        cancel its children.

        .. 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`.

        """
        return multi_future(children, quiet_exceptions=quiet_exceptions)

    Multi = multi

    def multi_future(
        children: Union[Sequence[_Yieldable], Mapping[Any, _Yieldable]],
        quiet_exceptions: "Union[Type[Exception], Tuple[Type[Exception], ...]]" = (),
    ) -> "Union[Future[List], Future[Dict]]":
        """Wait for multiple asynchronous futures in parallel.

        Since Tornado 6.0, this function is exactly the same as `multi`.

        .. 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())  # type: Optional[List]
            children_seq = children.values()  # type: Iterable
        else:
            keys = None
            children_seq = children
        children_futs = list(map(convert_yielded, children_seq))
        assert all(is_future(i) or isinstance(i, _NullFuture) for i in children_futs)
        unfinished_children = set(children_futs)

        future = _create_future()
        if not children_futs:
            future_set_result_unless_cancelled(future, {} if keys is not None else [])

        def callback(fut: Future) -> None:
            unfinished_children.remove(fut)
            if not unfinished_children:
                result_list = []
                for f in children_futs:
                    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(future, sys.exc_info())
                if not future.done():
                    if keys is not None:
                        future_set_result_unless_cancelled(
                            future, dict(zip(keys, result_list))
                        )
                    else:
                        future_set_result_unless_cancelled(future, result_list)

        listening = set()  # type: Set[Future]
        for f in children_futs:
            if f not in listening:
                listening.add(f)
                future_add_done_callback(f, callback)
        return future

    def maybe_future(x: Any) -> Future:
        """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 = _create_future()
            fut.set_result(x)
            return fut

    def with_timeout(
        timeout: Union[float, datetime.timedelta],
        future: _Yieldable,
        quiet_exceptions: "Union[Type[Exception], Tuple[Type[Exception], ...]]" = (),
    ) -> Future:
        """Wraps a `.Future` (or other yieldable object) in a timeout.

        Raises `tornado.util.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 either of a type contained in
        ``quiet_exceptions`` (which may be an exception type or a sequence of
        types), or an ``asyncio.CancelledError``.

        The wrapped `.Future` is not canceled when the timeout expires,
        permitting it to be reused. `asyncio.wait_for` is similar to this
        function but it does cancel the wrapped `.Future` on timeout.

        .. 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`.

        .. versionchanged:: 6.0.3
           ``asyncio.CancelledError`` is now always considered "quiet".

        .. versionchanged:: 6.2
           ``tornado.util.TimeoutError`` is now an alias to ``asyncio.TimeoutError``.

        """
        # 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_converted = convert_yielded(future)
        result = _create_future()
        chain_future(future_converted, result)
        io_loop = IOLoop.current()

        def error_callback(future: Future) -> None:
            try:
                future.result()
            except asyncio.CancelledError:
                pass
            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() -> None:
            if not result.done():
                result.set_exception(TimeoutError("Timeout"))
            # In case the wrapped future goes on to fail, log it.
            future_add_done_callback(future_converted, error_callback)

        timeout_handle = io_loop.add_timeout(timeout, timeout_callback)
        if isinstance(future_converted, 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(
                future_converted, 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_converted, lambda future: io_loop.remove_timeout(timeout_handle)
            )
        return result

    def sleep(duration: float) -> "Future[None]":
        """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 = _create_future()
        IOLoop.current().call_later(
            duration, lambda: future_set_result_unless_cancelled(f, None)
        )
        return f

    class _NullFuture:
        """_NullFuture resembles a Future that finished with a result of None.

        It's not actually a `Future` to avoid depending on a particular event loop.
        Handled as a special case in the coroutine runner.

        We lie and tell the type checker that a _NullFuture is a Future so
        we don't have to leak _NullFuture into lots of public APIs. But
        this means that the type checker can't warn us when we're passing
        a _NullFuture into a code path that doesn't understand what to do
        with it.
        """

        def result(self) -> None:
            return None

        def done(self) -> bool:
            return True

    # _null_future is used as a dummy value in the coroutine runner. It differs
    # from moment in that moment always adds a delay of one IOLoop iteration
    # while _null_future is processed as soon as possible.
    _null_future = typing.cast(Future, _NullFuture())

    moment = typing.cast(Future, _NullFuture())
    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``

    In native coroutines, the equivalent of ``yield gen.moment`` is
    ``await asyncio.sleep(0)``.

    .. versionadded:: 4.0

    .. deprecated:: 4.5
       ``yield None`` (or ``yield`` with no argument) is now equivalent to
        ``yield gen.moment``.
    """

    class Runner:
        """Internal implementation of `tornado.gen.coroutine`.

        Maintains information about pending callbacks and their results.

        The results of the generator are stored in ``result_future`` (a
        `.Future`)
        """

        def __init__(
            self,
            ctx_run: Callable,
            gen: "Generator[_Yieldable, Any, _T]",
            result_future: "Future[_T]",
            first_yielded: _Yieldable,
        ) -> None:
            self.ctx_run = ctx_run
            self.gen = gen
            self.result_future = result_future
            self.future = _null_future  # type: Union[None, Future]
            self.running = False
            self.finished = False
            self.io_loop = IOLoop.current()
            if self.ctx_run(self.handle_yield, first_yielded):
                gen = result_future = first_yielded = None  # type: ignore
                self.ctx_run(self.run)

        def run(self) -> None:
            """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 future is None:
                        raise Exception("No pending future")
                    if not future.done():
                        return
                    self.future = None
                    try:
                        try:
                            value = future.result()
                        except Exception as e:
                            # Save the exception for later. It's important that
                            # gen.throw() not be called inside this try/except block
                            # because that makes sys.exc_info behave unexpectedly.
                            exc: Optional[Exception] = e
                        else:
                            exc = None
                        finally:
                            future = None

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

                    except (StopIteration, Return) as e:
                        self.finished = True
                        self.future = _null_future
                        future_set_result_unless_cancelled(
                            self.result_future, _value_from_stopiteration(e)
                        )
                        self.result_future = None  # type: ignore
                        return
                    except Exception:
                        self.finished = True
                        self.future = _null_future
                        future_set_exc_info(self.result_future, sys.exc_info())
                        self.result_future = None  # type: ignore
                        return
                    if not self.handle_yield(yielded):
                        return
                    yielded = None
            finally:
                self.running = False

        def handle_yield(self, yielded: _Yieldable) -> bool:
            try:
                self.future = convert_yielded(yielded)
            except BadYieldError:
                self.future = Future()
                future_set_exc_info(self.future, sys.exc_info())

            if self.future is moment:
                self.io_loop.add_callback(self.ctx_run, self.run)
                return False
            elif self.future is None:
                raise Exception("no pending future")
            elif not self.future.done():

                def inner(f: Any) -> None:
                    # Break a reference cycle to speed GC.
                    f = None  # noqa: F841
                    self.ctx_run(self.run)

                self.io_loop.add_future(self.future, inner)
                return False
            return True

        def handle_exception(
            self, typ: Type[Exception], value: Exception, tb: types.TracebackType
        ) -> bool:
            if not self.running and not self.finished:
                self.future = Future()
                future_set_exc_info(self.future, (typ, value, tb))
                self.ctx_run(self.run)
                return True
            else:
                return False

    def _wrap_awaitable(awaitable: Awaitable) -> Future:
        # Convert Awaitables into Futures.
        # Note that we use ensure_future, which handles both awaitables
        # and coroutines, rather than create_task, which only accepts
        # coroutines. (ensure_future calls create_task if given a coroutine)
        fut = asyncio.ensure_future(awaitable)
        # See comments on IOLoop._pending_tasks.
        loop = IOLoop.current()
        loop._register_task(fut)
        fut.add_done_callback(lambda f: loop._unregister_task(f))
        return fut

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

        The default implementation accepts lists, dictionaries, and
        Futures. This has the side effect of starting any coroutines that
        did not start themselves, similar to `asyncio.ensure_future`.

        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 tornado.platform.asyncio.to_tornado_future(asyncio_future)

        .. versionadded:: 4.1

        """
        if yielded is None or yielded is moment:
            return moment
        elif yielded is _null_future:
            return _null_future
        elif isinstance(yielded, (list, dict)):
            return multi(yielded)  # type: ignore
        elif is_future(yielded):
            return typing.cast(Future, yielded)
        elif isawaitable(yielded):
            return _wrap_awaitable(yielded)  # type: ignore
        else:
            raise BadYieldError(f"yielded unknown object {yielded!r}")

    convert_yielded = singledispatch(convert_yielded)