# `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`

  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 tornado.ioloop
      import socket

      def connection_ready(sock, fd, events):
          while True:
              try:
                  connection, address = sock.accept()
              except socket.error as e:
                  if e.args[0] not in (errno.EWOULDBLOCK, errno.EAGAIN):
                      raise
                  return
              connection.setblocking(0)
              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.setblocking(0)
          sock.bind(("", port))
          sock.listen(128)

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

  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.

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

### Running an IOLoop

static `IOLoop.``current`(instance=True)

  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`, returns `IOLoop.instance()` (i.e. the main thread’s `IOLoop`, creating one if necessary) if `instance` is true.

  In general you should use `IOLoop.current` as the default when constructing an asynchronous object, and use `IOLoop.instance` when you mean to communicate to the main thread from a different one.

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

`IOLoop.``make_current`()

  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.``instance`()

  Returns a global `IOLoop` instance.

  Most applications have a single, global `IOLoop` running on the main thread. Use this method to get this instance from another thread. In most other cases, it is better to use `current()` to get the current thread’s `IOLoop`.

static `IOLoop.``initialized`()

  Returns true if the singleton instance has been created.

`IOLoop.``install`()

  Installs this `IOLoop` object as the singleton instance.

  This is normally not necessary as `instance()` will create an `IOLoop` on demand, but you may want to call `install` to use a custom subclass of `IOLoop`.

  When using an `IOLoop` subclass, `install` must be called prior to creating any objects that implicitly create their own `IOLoop` (e.g., `tornado.httpclient.AsyncHTTPClient`).

static `IOLoop.``clear_instance`()

  Clear the global `IOLoop` instance.

  New in version 4.0.

`IOLoop.``start`()

  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.

`IOLoop.``stop`()

  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()

  `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)

  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 `TimeoutError` is raised.

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

      @gen.coroutine
      def main():
          # do stuff...

      if __name__ == '__main__':
          IOLoop.current().run_sync(main)

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

`IOLoop.``close`(all_fds=False)

  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.

### I/O events

`IOLoop.``add_handler`(fd, handler, events)

  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)

  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.

`IOLoop.``remove_handler`(fd)

  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)

  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)

  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)

  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)

  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)

  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)

  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.

`IOLoop.``remove_timeout`(timeout)

  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)

  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.

`IOLoop.``time`()

  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, io_loop=None)

  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 4.1: The `io_loop` argument is deprecated.

  `start`()

    Starts the timer.

  `stop`()

    Stops the timer.

  `is_running`()

    Return True if this `PeriodicCallback` has been started.

    New in version 4.1.

### Debugging and error handling

`IOLoop.``handle_callback_exception`(callback)

  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)

  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.

`IOLoop.``set_blocking_log_threshold`(seconds)

  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)

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

  For use with `set_blocking_signal_threshold`.

### Methods for subclasses

`IOLoop.``initialize`(make_current=None)

`IOLoop.``close_fd`(fd)

  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.

`IOLoop.``split_fd`(fd)

  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.