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# `tornado.ioloop` — Main event loop
An I/O event loop for non-blocking sockets.
Typical applications will use a single [`IOLoop`](# "tornado.ioloop.IOLoop") object, in the[`IOLoop.instance`](# "tornado.ioloop.IOLoop.instance") singleton. The [`IOLoop.start`](# "tornado.ioloop.IOLoop.start") method should usuallybe called at the end of the `main()` function. Atypical applications mayuse more than one [`IOLoop`](# "tornado.ioloop.IOLoop"), such as one [`IOLoop`](# "tornado.ioloop.IOLoop") per thread, or per [`unittest`](https://docs.python.org/3.4/library/unittest.html#module-unittest "(in Python v3.4)") [https://docs.python.org/3.4/library/unittest.html#module-unittest]case.
In addition to I/O events, the [`IOLoop`](# "tornado.ioloop.IOLoop") can also schedule time-based events.[`IOLoop.add_timeout`](# "tornado.ioloop.IOLoop.add_timeout") is a non-blocking alternative to [`time.sleep`](https://docs.python.org/3.4/library/time.html#time.sleep "(in Python v3.4)") [https://docs.python.org/3.4/library/time.html#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 theyare available, or else we fall back on select(). If you areimplementing a system that needs to handle thousands ofsimultaneous connections, you should use a system that supportseither `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`](# "tornado.ioloop.IOLoop") becomes the thread's current[`IOLoop`](# "tornado.ioloop.IOLoop"), unless there already is a current [`IOLoop`](# "tornado.ioloop.IOLoop"). This behaviorcan be controlled with the `make_current` argument to the [`IOLoop`](# "tornado.ioloop.IOLoop")constructor: if `make_current=True`, the new [`IOLoop`](# "tornado.ioloop.IOLoop") will alwaystry to become current and it raises an error if there is already acurrent instance. If `make_current=False`, the new [`IOLoop`](# "tornado.ioloop.IOLoop") willnot try to become current.
Changed in version 4.2: Added the `make_current` keyword argument to the [`IOLoop`](# "tornado.ioloop.IOLoop")constructor.
### Running an IOLoop
*static *`IOLoop.``current`(*instance=True*)[[source]](#)
Returns the current thread's [`IOLoop`](# "tornado.ioloop.IOLoop").
If an [`IOLoop`](# "tornado.ioloop.IOLoop") is currently running or has been marked ascurrent by [`make_current`](# "tornado.ioloop.IOLoop.make_current"), returns that instance. If there isno current [`IOLoop`](# "tornado.ioloop.IOLoop"), returns [`IOLoop.instance()`](# "tornado.ioloop.IOLoop.instance") (i.e. themain thread's [`IOLoop`](# "tornado.ioloop.IOLoop"), creating one if necessary) if `instance`is true.
In general you should use [`IOLoop.current`](# "tornado.ioloop.IOLoop.current") as the default whenconstructing an asynchronous object, and use [`IOLoop.instance`](# "tornado.ioloop.IOLoop.instance")when you mean to communicate to the main thread from a differentone.
Changed in version 4.1: Added `instance` argument to control the fallback to[`IOLoop.instance()`](# "tornado.ioloop.IOLoop.instance").
`IOLoop.``make_current`()[[source]](#)
Makes this the [`IOLoop`](# "tornado.ioloop.IOLoop") for the current thread.
An [`IOLoop`](# "tornado.ioloop.IOLoop") automatically becomes current for its threadwhen it is started, but it is sometimes useful to call[`make_current`](# "tornado.ioloop.IOLoop.make_current") explicitly before starting the [`IOLoop`](# "tornado.ioloop.IOLoop"),so that code run at startup time can find the rightinstance.
Changed in version 4.1: An [`IOLoop`](# "tornado.ioloop.IOLoop") created while there is no current [`IOLoop`](# "tornado.ioloop.IOLoop")will automatically become current.
*static *`IOLoop.``instance`()[[source]](#)
Returns a global [`IOLoop`](# "tornado.ioloop.IOLoop") instance.
Most applications have a single, global [`IOLoop`](# "tornado.ioloop.IOLoop") running on themain thread. Use this method to get this instance fromanother thread. In most other cases, it is better to use [`current()`](# "tornado.ioloop.IOLoop.current")to get the current thread's [`IOLoop`](# "tornado.ioloop.IOLoop").
*static *`IOLoop.``initialized`()[[source]](#)
Returns true if the singleton instance has been created.
`IOLoop.``install`()[[source]](#)
Installs this [`IOLoop`](# "tornado.ioloop.IOLoop") object as the singleton instance.
This is normally not necessary as [`instance()`](# "tornado.ioloop.IOLoop.instance") will createan [`IOLoop`](# "tornado.ioloop.IOLoop") on demand, but you may want to call [`install`](# "tornado.ioloop.IOLoop.install") to usea custom subclass of [`IOLoop`](# "tornado.ioloop.IOLoop").
*static *`IOLoop.``clear_instance`()[[source]](#)
Clear the global [`IOLoop`](# "tornado.ioloop.IOLoop") instance.
New in version 4.0.
`IOLoop.``start`()[[source]](#)
Starts the I/O loop.
The loop will run until one of the callbacks calls [`stop()`](# "tornado.ioloop.IOLoop.stop"), whichwill make the loop stop after the current event iteration completes.
`IOLoop.``stop`()[[source]](#)
Stop the I/O loop.
If the event loop is not currently running, the next call to [`start()`](# "tornado.ioloop.IOLoop.start")will return immediately.
To use asynchronous methods from otherwise-synchronous code (such asunit 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 runits callback, whether that callback was invoked before orafter `ioloop.start`.
Note that even after [`stop`](# "tornado.ioloop.IOLoop.stop") has been called, the [`IOLoop`](# "tornado.ioloop.IOLoop") is notcompletely stopped until [`IOLoop.start`](# "tornado.ioloop.IOLoop.start") has also returned.Some work that was scheduled before the call to [`stop`](# "tornado.ioloop.IOLoop.stop") may stillbe run before the [`IOLoop`](# "tornado.ioloop.IOLoop") shuts down.
`IOLoop.``run_sync`(*func*, *timeout=None*)[[source]](#)
Starts the [`IOLoop`](# "tornado.ioloop.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`](# "tornado.ioloop.IOLoop") will run until the yieldable is resolved (and[`run_sync()`](# "tornado.ioloop.IOLoop.run_sync") will return the yieldable's result). If it raisesan exception, the [`IOLoop`](# "tornado.ioloop.IOLoop") will stop and the exception will bere-raised to the caller.
The keyword-only argument `timeout` may be used to seta maximum duration for the function. If the timeout expires,a [`TimeoutError`](https://docs.python.org/3.4/library/exceptions.html#TimeoutError "(in Python v3.4)") [https://docs.python.org/3.4/library/exceptions.html#TimeoutError] is raised.
This method is useful in conjunction with [`tornado.gen.coroutine`](# "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*)[[source]](#)
Closes the [`IOLoop`](# "tornado.ioloop.IOLoop"), freeing any resources used.
If `all_fds` is true, all file descriptors registered on theIOLoop will be closed (not just the ones created by the[`IOLoop`](# "tornado.ioloop.IOLoop") itself).
Many applications will only use a single [`IOLoop`](# "tornado.ioloop.IOLoop") that runs for theentire lifetime of the process. In that case closing the [`IOLoop`](# "tornado.ioloop.IOLoop")is not necessary since everything will be cleaned up when theprocess exits. [`IOLoop.close`](# "tornado.ioloop.IOLoop.close") is provided mainly for scenariossuch as unit tests, which create and destroy a large number of`IOLoops`.
An [`IOLoop`](# "tornado.ioloop.IOLoop") must be completely stopped before it can be closed. Thismeans that [`IOLoop.stop()`](# "tornado.ioloop.IOLoop.stop") must be called *and*[`IOLoop.start()`](# "tornado.ioloop.IOLoop.start") mustbe allowed to return before attempting to call [`IOLoop.close()`](# "tornado.ioloop.IOLoop.close").Therefore the call to [`close`](# "tornado.ioloop.IOLoop.close") will usually appear just afterthe call to [`start`](# "tornado.ioloop.IOLoop.start") rather than near the call to [`stop`](# "tornado.ioloop.IOLoop.stop").
Changed in version 3.1: If the [`IOLoop`](# "tornado.ioloop.IOLoop") implementation supports non-integer objectsfor “file descriptors”, those objects will have their`close` method when `all_fds` is true.
### 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 ora file-like object with a `fileno()` method (and optionally a`close()` method, which may be called when the [`IOLoop`](# "tornado.ioloop.IOLoop") is shutdown).
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 toraw 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 toraw file descriptors.
`IOLoop.``remove_handler`(*fd*)[[source]](#)
Stop listening for events on `fd`.
Changed in version 4.0: Added the ability to pass file-like objects in addition toraw 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`](# "tornado.ioloop.IOLoop") that makes this thread-safety guarantee; allother interaction with the [`IOLoop`](# "tornado.ioloop.IOLoop") must be done from that[`IOLoop`](# "tornado.ioloop.IOLoop")‘s thread. [`add_callback()`](# "tornado.ioloop.IOLoop.add_callback") may be used to transfercontrol from other threads to the [`IOLoop`](# "tornado.ioloop.IOLoop")‘s thread.
To add a callback from a signal handler, see[`add_callback_from_signal`](# "tornado.ioloop.IOLoop.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 usedotherwise.
Callbacks added with this method will be run without any[`stack_context`](# "tornado.stack_context"), to avoid picking up the context of the functionthat was interrupted by the signal.
`IOLoop.``add_future`(*future*, *callback*)[[source]](#)
Schedules a callback on the `IOLoop` when the given[`Future`](# "tornado.concurrent.Future") is finished.
The callback is invoked with one argument, the[`Future`](# "tornado.concurrent.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`](# "tornado.ioloop.IOLoop.remove_timeout") to cancel.
`deadline` may be a number denoting a time (on the samescale as [`IOLoop.time`](# "tornado.ioloop.IOLoop.time"), normally [`time.time`](https://docs.python.org/3.4/library/time.html#time.time "(in Python v3.4)") [https://docs.python.org/3.4/library/time.html#time.time]), or a[`datetime.timedelta`](https://docs.python.org/3.4/library/datetime.html#datetime.timedelta "(in Python v3.4)") [https://docs.python.org/3.4/library/datetime.html#datetime.timedelta] object for a deadline relative to thecurrent time. Since Tornado 4.0, [`call_later`](# "tornado.ioloop.IOLoop.call_later") is a moreconvenient alternative for the relative case since it does notrequire a timedelta object.
Note that it is not safe to call [`add_timeout`](# "tornado.ioloop.IOLoop.add_timeout") from other threads.Instead, you must use [`add_callback`](# "tornado.ioloop.IOLoop.add_callback") to transfer control to the[`IOLoop`](# "tornado.ioloop.IOLoop")‘s thread, and then call [`add_timeout`](# "tornado.ioloop.IOLoop.add_timeout") from there.
Subclasses of IOLoop must implement either [`add_timeout`](# "tornado.ioloop.IOLoop.add_timeout") or[`call_at`](# "tornado.ioloop.IOLoop.call_at"); the default implementations of each will callthe other. [`call_at`](# "tornado.ioloop.IOLoop.call_at") is usually easier to implement, butsubclasses that wish to maintain compatibility with Tornadoversions prior to 4.0 must use [`add_timeout`](# "tornado.ioloop.IOLoop.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`](# "tornado.ioloop.IOLoop.time").
Returns an opaque handle that may be passed to [`remove_timeout`](# "tornado.ioloop.IOLoop.remove_timeout")to cancel. Note that unlike the [`asyncio`](https://docs.python.org/3.4/library/asyncio.html#module-asyncio "(in Python v3.4)") [https://docs.python.org/3.4/library/asyncio.html#module-asyncio] method of the samename, the returned object does not have a `cancel()` method.
See [`add_timeout`](# "tornado.ioloop.IOLoop.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`](# "tornado.ioloop.IOLoop.remove_timeout")to cancel. Note that unlike the [`asyncio`](https://docs.python.org/3.4/library/asyncio.html#module-asyncio "(in Python v3.4)") [https://docs.python.org/3.4/library/asyncio.html#module-asyncio] method of the samename, the returned object does not have a `cancel()` method.
See [`add_timeout`](# "tornado.ioloop.IOLoop.add_timeout") for comments on thread-safety and subclassing.
New in version 4.0.
`IOLoop.``remove_timeout`(*timeout*)[[source]](#)
Cancels a pending timeout.
The argument is a handle as returned by [`add_timeout`](# "tornado.ioloop.IOLoop.add_timeout"). It issafe to call [`remove_timeout`](# "tornado.ioloop.IOLoop.remove_timeout") even if the callback has alreadybeen 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 callbacksthat should not interfere with the caller.
New in version 4.0.
`IOLoop.``time`()[[source]](#)
Returns the current time according to the [`IOLoop`](# "tornado.ioloop.IOLoop")‘s clock.
The return value is a floating-point number relative to anunspecified time in the past.
By default, the [`IOLoop`](# "tornado.ioloop.IOLoop")‘s time function is [`time.time`](https://docs.python.org/3.4/library/time.html#time.time "(in Python v3.4)") [https://docs.python.org/3.4/library/time.html#time.time]. However,it may be configured to use e.g. [`time.monotonic`](https://docs.python.org/3.4/library/time.html#time.monotonic "(in Python v3.4)") [https://docs.python.org/3.4/library/time.html#time.monotonic] instead.Calls to [`add_timeout`](# "tornado.ioloop.IOLoop.add_timeout") that pass a number instead of a[`datetime.timedelta`](https://docs.python.org/3.4/library/datetime.html#datetime.timedelta "(in Python v3.4)") [https://docs.python.org/3.4/library/datetime.html#datetime.timedelta] should use this function to compute theappropriate time, so they can work no matter what time functionis chosen.
*class *`tornado.ioloop.``PeriodicCallback`(*callback*, *callback_time*, *io_loop=None*)[[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 othertime-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`](# "tornado.ioloop.PeriodicCallback.start") must be called after the [`PeriodicCallback`](# "tornado.ioloop.PeriodicCallback") is created.
Changed in version 4.1: The `io_loop` argument is deprecated.
`start`()[[source]](#)
Starts the timer.
`stop`()[[source]](#)
Stops the timer.
`is_running`()[[source]](#)
Return True if this [`PeriodicCallback`](# "tornado.ioloop.PeriodicCallback") has been started.
New in version 4.1.
### Debugging and error handling
`IOLoop.``handle_callback_exception`(*callback*)[[source]](#)
This method is called whenever a callback run by the [`IOLoop`](# "tornado.ioloop.IOLoop")throws an exception.
By default simply logs the exception as an error. Subclassesmay override this method to customize reporting of exceptions.
The exception itself is not passed explicitly, but is availablein [`sys.exc_info`](https://docs.python.org/3.4/library/sys.html#sys.exc_info "(in Python v3.4)") [https://docs.python.org/3.4/library/sys.html#sys.exc_info].
`IOLoop.``set_blocking_signal_threshold`(*seconds*, *action*)[[source]](#)
Sends a signal if the [`IOLoop`](# "tornado.ioloop.IOLoop") is blocked for more than`s` seconds.
Pass `seconds=None` to disable. Requires Python 2.6 on a unixyplatform.
The action parameter is a Python signal handler. Read thedocumentation for the [`signal`](https://docs.python.org/3.4/library/signal.html#module-signal "(in Python v3.4)") [https://docs.python.org/3.4/library/signal.html#module-signal] module for more information.If `action` is None, the process will be killed if it isblocked for too long.
`IOLoop.``set_blocking_log_threshold`(*seconds*)[[source]](#)
Logs a stack trace if the [`IOLoop`](# "tornado.ioloop.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`](# "tornado.ioloop.IOLoop.set_blocking_signal_threshold").
### Methods for subclasses
`IOLoop.``initialize`(*make_current=None*)[[source]](#)`IOLoop.``close_fd`(*fd*)[[source]](#)
Utility method to close an `fd`.
If `fd` is a file-like object, we close it directly; otherwisewe use [`os.close`](https://docs.python.org/3.4/library/os.html#os.close "(in Python v3.4)") [https://docs.python.org/3.4/library/os.html#os.close].
This method is provided for use by [`IOLoop`](# "tornado.ioloop.IOLoop") subclasses (inimplementations of `IOLoop.close(all_fds=True)` and shouldnot generally be used by application code.
New in version 4.0.
`IOLoop.``split_fd`(*fd*)[[source]](#)
Returns an (fd, obj) pair from an `fd` parameter.
We accept both raw file descriptors and file-like objects asinput to [`add_handler`](# "tornado.ioloop.IOLoop.add_handler") and related methods. When a file-likeobject is passed, we must retain the object itself so we canclose it correctly when the [`IOLoop`](# "tornado.ioloop.IOLoop") shuts down, but thepoller interfaces favor file descriptors (they will acceptfile-like objects and call `fileno()` for you, but theyalways return the descriptor itself).
This method is provided for use by [`IOLoop`](# "tornado.ioloop.IOLoop") subclasses and shouldnot generally be used by application code.
New in version 4.0.
© Copyright 2009-2016, The Tornado Authors. Created using [Sphinx](http://sphinx-doc.org/) 1.3.5.
- User's guide
- Introduction
- Asynchronous and non-Blocking I/O
- Coroutines
- Queue example - a concurrent web spider
- Structure of a Tornado web application
- Templates and UI
- Authentication and security
- Running and deploying
- Web framework
- tornado.web — RequestHandler and Application classes
- tornado.template — Flexible output generation
- tornado.escape — Escaping and string manipulation
- tornado.locale — Internationalization support
- tornado.websocket — Bidirectional communication to the browser
- HTTP servers and clients
- tornado.httpserver — Non-blocking HTTP server
- tornado.httpclient — Asynchronous HTTP client
- tornado.httputil — Manipulate HTTP headers and URLs
- tornado.http1connection – HTTP/1.x client/server implementation
- Asynchronous networking
- tornado.ioloop — Main event loop
- tornado.iostream — Convenient wrappers for non-blocking sockets
- tornado.netutil — Miscellaneous network utilities
- tornado.tcpclient — IOStream connection factory
- tornado.tcpserver — Basic IOStream-based TCP server
- Coroutines and concurrency
- tornado.gen — Simplify asynchronous code
- tornado.concurrent — Work with threads and futures
- tornado.locks – Synchronization primitives
- tornado.queues – Queues for coroutines
- tornado.process — Utilities for multiple processes
- Integration with other services
- tornado.auth — Third-party login with OpenID and OAuth
- tornado.wsgi — Interoperability with other Python frameworks and servers
- tornado.platform.asyncio — Bridge between asyncio and Tornado
- tornado.platform.caresresolver — Asynchronous DNS Resolver using C-Ares
- tornado.platform.twisted — Bridges between Twisted and Tornado
- Utilities
- tornado.autoreload — Automatically detect code changes in development
- tornado.log — Logging support
- tornado.options — Command-line parsing
- tornado.stack_context — Exception handling across asynchronous callbacks
- tornado.testing — Unit testing support for asynchronous code
- tornado.util — General-purpose utilities
- Frequently Asked Questions
- Release notes
- What's new in Tornado 4.3
- What's new in Tornado 4.2.1
- What's new in Tornado 4.2
- What's new in Tornado 4.1
- What's new in Tornado 4.0.2
- What's new in Tornado 4.0.1
- What's new in Tornado 4.0
- What's new in Tornado 3.2.2
- What's new in Tornado 3.2.1
- What's new in Tornado 3.2
- What's new in Tornado 3.1.1
- What's new in Tornado 3.1
- What's new in Tornado 3.0.2
- What's new in Tornado 3.0.1
- What's new in Tornado 3.0
- What's new in Tornado 2.4.1
- What's new in Tornado 2.4
- What's new in Tornado 2.3
- What's new in Tornado 2.2.1
- What's new in Tornado 2.2
- What's new in Tornado 2.1.1
- What's new in Tornado 2.1
- What's new in Tornado 2.0
- What's new in Tornado 1.2.1
- What's new in Tornado 1.2
- What's new in Tornado 1.1.1
- What's new in Tornado 1.1
- What's new in Tornado 1.0.1
- What's new in Tornado 1.0