tornado.ioloop — Main event loop

An I/O event loop for non-blocking sockets.

Typical applications will use a single IOLoop object, in the IOLoop.instance singleton. The IOLoop.start method should usually be called at the end of the main() function. Atypical applications may use more than one IOLoop, such as one IOLoop per thread, or per unittest case.

In addition to I/O events, the IOLoop can also schedule time-based events. IOLoop.add_timeout is a non-blocking alternative to time.sleep.

IOLoop objects

class tornado.ioloop.IOLoop[source]

A level-triggered I/O loop.

We use epoll (Linux) or kqueue (BSD and Mac OS X) if they are available, or else we fall back on select(). If you are implementing a system that needs to handle thousands of simultaneous connections, you should use a system that supports either epoll or kqueue.

Example usage for a simple TCP server:

import errno
import functools
import socket

import tornado.ioloop
from tornado import gen
from tornado.iostream import IOStream

def handle_connection(connection, address):
    stream = IOStream(connection)
    message = yield stream.read_until_close()
    print("message from client:", message.decode().strip())

def connection_ready(sock, fd, events):
    while True:
            connection, address = sock.accept()
        except socket.error as e:
            if e.args[0] not in (errno.EWOULDBLOCK, errno.EAGAIN):
        handle_connection(connection, address)

if __name__ == '__main__':
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0)
    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    sock.bind(("", 8888))

    io_loop = tornado.ioloop.IOLoop.current()
    callback = functools.partial(connection_ready, sock)
    io_loop.add_handler(sock.fileno(), callback, io_loop.READ)

By default, a newly-constructed IOLoop becomes the thread’s current IOLoop, unless there already is a current IOLoop. This behavior can be controlled with the make_current argument to the IOLoop constructor: if make_current=True, the new IOLoop will always try to become current and it raises an error if there is already a current instance. If make_current=False, the new IOLoop will not try to become current.

In general, an IOLoop cannot survive a fork or be shared across processes in any way. When multiple processes are being used, each process should create its own IOLoop, which also implies that any objects which depend on the IOLoop (such as AsyncHTTPClient) must also be created in the child processes. As a guideline, anything that starts processes (including the tornado.process and multiprocessing modules) should do so as early as possible, ideally the first thing the application does after loading its configuration in main().

Changed in version 4.2: Added the make_current keyword argument to the IOLoop constructor.

Running an IOLoop

static IOLoop.current(instance=True)[source]

Returns the current thread’s IOLoop.

If an IOLoop is currently running or has been marked as current by make_current, returns that instance. If there is no current IOLoop and instance is true, creates one.

Changed in version 4.1: Added instance argument to control the fallback to IOLoop.instance().

Changed in version 5.0: The instance argument now controls whether an IOLoop is created automatically when there is none, instead of whether we fall back to IOLoop.instance() (which is now an alias for this method)


Makes this the IOLoop for the current thread.

An IOLoop automatically becomes current for its thread when it is started, but it is sometimes useful to call make_current explicitly before starting the IOLoop, so that code run at startup time can find the right instance.

Changed in version 4.1: An IOLoop created while there is no current IOLoop will automatically become current.

static IOLoop.clear_current()[source]

Clears the IOLoop for the current thread.

Intended primarily for use by test frameworks in between tests.


Starts the I/O loop.

The loop will run until one of the callbacks calls stop(), which will make the loop stop after the current event iteration completes.


Stop the I/O loop.

If the event loop is not currently running, the next call to start() will return immediately.

To use asynchronous methods from otherwise-synchronous code (such as unit tests), you can start and stop the event loop like this:

ioloop = IOLoop()
async_method(ioloop=ioloop, callback=ioloop.stop)

ioloop.start() will return after async_method has run its callback, whether that callback was invoked before or after ioloop.start.

Note that even after stop has been called, the IOLoop is not completely stopped until IOLoop.start has also returned. Some work that was scheduled before the call to stop may still be run before the IOLoop shuts down.

IOLoop.run_sync(func, timeout=None)[source]

Starts the IOLoop, runs the given function, and stops the loop.

The function must return either a yieldable object or None. If the function returns a yieldable object, the IOLoop will run until the yieldable is resolved (and run_sync() will return the yieldable’s result). If it raises an exception, the IOLoop will stop and the exception will be re-raised to the caller.

The keyword-only argument timeout may be used to set a maximum duration for the function. If the timeout expires, a tornado.util.TimeoutError is raised.

This method is useful in conjunction with tornado.gen.coroutine to allow asynchronous calls in a main() function:

def main():
    # do stuff...

if __name__ == '__main__':

Changed in version 4.3: Returning a non-None, non-yieldable value is now an error.


Closes the IOLoop, freeing any resources used.

If all_fds is true, all file descriptors registered on the IOLoop will be closed (not just the ones created by the IOLoop itself).

Many applications will only use a single IOLoop that runs for the entire lifetime of the process. In that case closing the IOLoop is not necessary since everything will be cleaned up when the process exits. IOLoop.close is provided mainly for scenarios such as unit tests, which create and destroy a large number of IOLoops.

An IOLoop must be completely stopped before it can be closed. This means that IOLoop.stop() must be called and IOLoop.start() must be allowed to return before attempting to call IOLoop.close(). Therefore the call to close will usually appear just after the call to start rather than near the call to stop.

Changed in version 3.1: If the IOLoop implementation supports non-integer objects for “file descriptors”, those objects will have their close method when all_fds is true.

static IOLoop.instance()[source]

Deprecated alias for IOLoop.current().

Changed in version 5.0: Previously, this method returned a global singleton IOLoop, in contrast with the per-thread IOLoop returned by current(). In nearly all cases the two were the same (when they differed, it was generally used from non-Tornado threads to communicate back to the main thread’s IOLoop). This distinction is not present in asyncio, so in order to facilitate integration with that package instance() was changed to be an alias to current(). Applications using the cross-thread communications aspect of instance() should instead set their own global variable to point to the IOLoop they want to use.

Deprecated since version 5.0.

static IOLoop.initialized()[source]

Returns true if there is a current IOLoop.

Changed in version 5.0: Redefined in terms of current() instead of instance().

Deprecated since version 5.0: This method only knows about IOLoop objects (and not, for example, asyncio event loops), so it is of limited use.


Deprecated alias for make_current().

Changed in version 5.0: Previously, this method would set this IOLoop as the global singleton used by IOLoop.instance(). Now that instance() is an alias for current(), install() is an alias for make_current().

Deprecated since version 5.0.

static IOLoop.clear_instance()[source]

Deprecated alias for clear_current().

Changed in version 5.0: Previously, this method would clear the IOLoop used as the global singleton by IOLoop.instance(). Now that instance() is an alias for current(), clear_instance() is an alias for clear_instance().

Deprecated since version 5.0.

I/O events

IOLoop.add_handler(fd, handler, events)[source]

Registers the given handler to receive the given events for fd.

The fd argument may either be an integer file descriptor or a file-like object with a fileno() method (and optionally a close() method, which may be called when the IOLoop is shut down).

The events argument is a bitwise or of the constants IOLoop.READ, IOLoop.WRITE, and IOLoop.ERROR.

When an event occurs, handler(fd, events) will be run.

Changed in version 4.0: Added the ability to pass file-like objects in addition to raw file descriptors.

IOLoop.update_handler(fd, events)[source]

Changes the events we listen for fd.

Changed in version 4.0: Added the ability to pass file-like objects in addition to raw file descriptors.


Stop listening for events on fd.

Changed in version 4.0: Added the ability to pass file-like objects in addition to raw file descriptors.

Callbacks and timeouts

IOLoop.add_callback(callback, *args, **kwargs)[source]

Calls the given callback on the next I/O loop iteration.

It is safe to call this method from any thread at any time, except from a signal handler. Note that this is the only method in IOLoop that makes this thread-safety guarantee; all other interaction with the IOLoop must be done from that IOLoop’s thread. add_callback() may be used to transfer control from other threads to the IOLoop’s thread.

To add a callback from a signal handler, see add_callback_from_signal.

IOLoop.add_callback_from_signal(callback, *args, **kwargs)[source]

Calls the given callback on the next I/O loop iteration.

Safe for use from a Python signal handler; should not be used otherwise.

Callbacks added with this method will be run without any stack_context, to avoid picking up the context of the function that was interrupted by the signal.

IOLoop.add_future(future, callback)[source]

Schedules a callback on the IOLoop when the given Future is finished.

The callback is invoked with one argument, the Future.

IOLoop.add_timeout(deadline, callback, *args, **kwargs)[source]

Runs the callback at the time deadline from the I/O loop.

Returns an opaque handle that may be passed to remove_timeout to cancel.

deadline may be a number denoting a time (on the same scale as IOLoop.time, normally time.time), or a datetime.timedelta object for a deadline relative to the current time. Since Tornado 4.0, call_later is a more convenient alternative for the relative case since it does not require a timedelta object.

Note that it is not safe to call add_timeout from other threads. Instead, you must use add_callback to transfer control to the IOLoop’s thread, and then call add_timeout from there.

Subclasses of IOLoop must implement either add_timeout or call_at; the default implementations of each will call the other. call_at is usually easier to implement, but subclasses that wish to maintain compatibility with Tornado versions prior to 4.0 must use add_timeout instead.

Changed in version 4.0: Now passes through *args and **kwargs to the callback.

IOLoop.call_at(when, callback, *args, **kwargs)[source]

Runs the callback at the absolute time designated by when.

when must be a number using the same reference point as IOLoop.time.

Returns an opaque handle that may be passed to remove_timeout to cancel. Note that unlike the asyncio method of the same name, the returned object does not have a cancel() method.

See add_timeout for comments on thread-safety and subclassing.

New in version 4.0.

IOLoop.call_later(delay, callback, *args, **kwargs)[source]

Runs the callback after delay seconds have passed.

Returns an opaque handle that may be passed to remove_timeout to cancel. Note that unlike the asyncio method of the same name, the returned object does not have a cancel() method.

See add_timeout for comments on thread-safety and subclassing.

New in version 4.0.


Cancels a pending timeout.

The argument is a handle as returned by add_timeout. It is safe to call remove_timeout even if the callback has already been run.

IOLoop.spawn_callback(callback, *args, **kwargs)[source]

Calls the given callback on the next IOLoop iteration.

Unlike all other callback-related methods on IOLoop, spawn_callback does not associate the callback with its caller’s stack_context, so it is suitable for fire-and-forget callbacks that should not interfere with the caller.

New in version 4.0.


Returns the current time according to the IOLoop’s clock.

The return value is a floating-point number relative to an unspecified time in the past.

By default, the IOLoop’s time function is time.time. However, it may be configured to use e.g. time.monotonic instead. Calls to add_timeout that pass a number instead of a datetime.timedelta should use this function to compute the appropriate time, so they can work no matter what time function is chosen.

class tornado.ioloop.PeriodicCallback(callback, callback_time)[source]

Schedules the given callback to be called periodically.

The callback is called every callback_time milliseconds. Note that the timeout is given in milliseconds, while most other time-related functions in Tornado use seconds.

If the callback runs for longer than callback_time milliseconds, subsequent invocations will be skipped to get back on schedule.

start must be called after the PeriodicCallback is created.

Changed in version 5.0: The io_loop argument (deprecated since version 4.1) has been removed.


Starts the timer.


Stops the timer.


Return True if this PeriodicCallback has been started.

New in version 4.1.

Debugging and error handling


This method is called whenever a callback run by the IOLoop throws an exception.

By default simply logs the exception as an error. Subclasses may override this method to customize reporting of exceptions.

The exception itself is not passed explicitly, but is available in sys.exc_info.

IOLoop.set_blocking_signal_threshold(seconds, action)[source]

Sends a signal if the IOLoop is blocked for more than s seconds.

Pass seconds=None to disable. Requires Python 2.6 on a unixy platform.

The action parameter is a Python signal handler. Read the documentation for the signal module for more information. If action is None, the process will be killed if it is blocked for too long.


Logs a stack trace if the IOLoop is blocked for more than s seconds.

Equivalent to set_blocking_signal_threshold(seconds, self.log_stack)

IOLoop.log_stack(signal, frame)[source]

Signal handler to log the stack trace of the current thread.

For use with set_blocking_signal_threshold.

Methods for subclasses


Utility method to close an fd.

If fd is a file-like object, we close it directly; otherwise we use os.close.

This method is provided for use by IOLoop subclasses (in implementations of IOLoop.close(all_fds=True) and should not generally be used by application code.

New in version 4.0.


Returns an (fd, obj) pair from an fd parameter.

We accept both raw file descriptors and file-like objects as input to add_handler and related methods. When a file-like object is passed, we must retain the object itself so we can close it correctly when the IOLoop shuts down, but the poller interfaces favor file descriptors (they will accept file-like objects and call fileno() for you, but they always return the descriptor itself).

This method is provided for use by IOLoop subclasses and should not generally be used by application code.

New in version 4.0.