tornado.gen — Simplify asynchronous code

tornado.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:

class AsyncHandler(RequestHandler):
    def get(self):
        http_client = AsyncHTTPClient()

    def on_fetch(self, response):

could be written with gen as:

class GenAsyncHandler(RequestHandler):
    def get(self):
        http_client = AsyncHTTPClient()
        response = yield http_client.fetch("")

Most asynchronous functions in Tornado return a Future; yielding this object returns its 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:

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

If the singledispatch library is available (standard in Python 3.4, available via the singledispatch package on older versions), additional types of objects may be yielded. Tornado includes support for asyncio.Future and Twisted’s Deferred class when tornado.platform.asyncio and tornado.platform.twisted are imported. See the convert_yielded function to extend this mechanism.

Changed in version 3.2: Dict support added.

Changed in version 4.1: Support added for yielding asyncio Futures and Twisted Deferreds via singledispatch.


tornado.gen.coroutine(func, replace_callback=True)[source]

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). 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.


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.


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 RequestHandler HTTP verb methods, which use self.finish() in place of a callback argument.

Utility functions

exception tornado.gen.Return(value=None)[source]

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:

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.

tornado.gen.with_timeout(timeout, future, io_loop=None, quiet_exceptions=())[source]

Wraps a Future 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).

Currently only supports Futures, not other YieldPoint classes.

New in version 4.0.

Changed in version 4.1: Added the quiet_exceptions argument and the logging of unhandled exceptions.

exception tornado.gen.TimeoutError[source]

Exception raised by with_timeout.


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).

New in version 4.1.


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

New in version 4.0.

class tornado.gen.WaitIterator(*args, **kwargs)[source]

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():
        result = yield
    except Exception as e:
        print("Error {} from {}".format(e, wait_iterator.current_future))
        print("Result {} received from {} at {}".format(
            result, wait_iterator.current_future,

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,

New in version 4.1.

Changed in version 4.3: Added async for support in Python 3.5.


Returns True if this iterator has no more results.


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.

tornado.gen.multi(children, quiet_exceptions=())[source]

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, 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.

Changed in version 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.

Changed in version 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.

tornado.gen.multi_future(children, quiet_exceptions=())[source]

Wait for multiple asynchronous futures in parallel.

This function is similar to multi, but does not support YieldPoints.

New in version 4.0.

Changed in version 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 since version 4.3: Use multi instead.

tornado.gen.Task(func, *args, **kwargs)[source]

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.

Changed in version 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.

class tornado.gen.Arguments

The result of a Task or Wait whose callback had more than one argument (or keyword arguments).

The Arguments object is a collections.namedtuple and can be used either as a tuple (args, kwargs) or an object with attributes args and kwargs.


Convert a yielded object into a Future.

The default implementation accepts lists, dictionaries, and Futures.

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

def _(asyncio_future):
    return tornado.platform.asyncio.to_tornado_future(asyncio_future)

New in version 4.1.


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 since version 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.

Legacy interface

Before support for Futures was introduced in Tornado 3.0, coroutines used subclasses of YieldPoint in their yield expressions. These classes are still supported but should generally not be used except for compatibility with older interfaces. None of these classes are compatible with native (await-based) coroutines.

class tornado.gen.YieldPoint[source]

Base class for objects that may be yielded from the generator.

Deprecated since version 4.0: Use Futures instead.


Called by the runner after the generator has yielded.

No other methods will be called on this object before start.


Called by the runner to determine whether to resume the generator.

Returns a boolean; may be called more than once.


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.

class tornado.gen.Callback(key)[source]

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 since version 4.0: Use Futures instead.

class tornado.gen.Wait(key)[source]

Returns the argument passed to the result of a previous Callback.

Deprecated since version 4.0: Use Futures instead.

class tornado.gen.WaitAll(keys)[source]

Returns the results of multiple previous Callbacks.

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 since version 4.0: Use Futures instead.

class tornado.gen.MultiYieldPoint(children, quiet_exceptions=())[source]

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.

Changed in version 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.

Changed in version 4.3: Renamed from Multi to MultiYieldPoint. The name Multi remains as an alias for the equivalent multi function.

Deprecated since version 4.3: Use multi instead.