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+.. module:: asyncio
+
+.. _event-loop:
+
+Event loops
+===========
+
+The event loop is the central execution device provided by :mod:`asyncio`.
+It provides multiple facilities, amongst which:
+
+* Registering, executing and cancelling delayed calls (timeouts)
+
+* Creating client and server :ref:`transports <transport>` for various
+  kinds of communication
+
+* Launching subprocesses and the associated :ref:`transports <transport>`
+  for communication with an external program
+
+* Delegating costly function calls to a pool of threads
+
+Event loop functions
+--------------------
+
+The easiest way to get an event loop is to call the :func:`get_event_loop`
+function.
+
+.. function:: get_event_loop()
+
+   Get the event loop for current context. Returns an event loop object
+   implementing :class:`BaseEventLoop` interface, or raises an exception in case no
+   event loop has been set for the current context and the current policy does
+   not specify to create one. It should never return ``None``.
+
+.. function:: set_event_loop(loop)
+
+   XXX
+
+.. function:: new_event_loop()
+
+   XXX
+
+
+Event loop policy
+-----------------
+
+.. function:: get_event_loop_policy()
+
+   XXX
+
+.. function:: set_event_loop_policy(policy)
+
+   XXX
+
+
+Run an event loop
+-----------------
+
+.. method:: BaseEventLoop.run_forever()
+
+   Run until :meth:`stop` is called.
+
+.. method:: BaseEventLoop.run_until_complete(future)
+
+   Run until the :class:`~concurrent.futures.Future` is done.
+
+   If the argument is a coroutine, it is wrapped in a :class:`Task`.
+
+   Return the Future's result, or raise its exception.
+
+.. method:: BaseEventLoop.is_running()
+
+   Returns running status of event loop.
+
+.. method:: stop()
+
+   Stop running the event loop.
+
+   Every callback scheduled before :meth:`stop` is called will run.
+   Callback scheduled after :meth:`stop` is called won't.  However, those
+   callbacks will run if :meth:`run_forever` is called again later.
+
+.. method:: BaseEventLoop.close()
+
+   Close the event loop. The loop should not be running.
+
+   This clears the queues and shuts down the executor, but does not wait for
+   the executor to finish.
+
+   This is idempotent and irreversible. No other methods should be called after
+   this one.
+
+
+Calls
+-----
+
+.. method:: BaseEventLoop.call_soon(callback, \*args)
+
+   Arrange for a callback to be called as soon as possible.
+
+   This operates as a FIFO queue, callbacks are called in the order in
+   which they are registered.  Each callback will be called exactly once.
+
+   Any positional arguments after the callback will be passed to the
+   callback when it is called.
+
+.. method:: BaseEventLoop.call_soon_threadsafe(callback, \*args)
+
+   Like :meth:`call_soon`, but thread safe.
+
+
+Delayed calls
+-------------
+
+The event loop has its own internal clock for computing timeouts.
+Which clock is used depends on the (platform-specific) event loop
+implementation; ideally it is a monotonic clock.  This will generally be
+a different clock than :func:`time.time`.
+
+.. method:: BaseEventLoop.call_later(delay, callback, *args)
+
+   Arrange for the *callback* to be called after the given *delay*
+   seconds (either an int or float).
+
+   A "handle" is returned: an opaque object with a :meth:`cancel` method
+   that can be used to cancel the call.
+
+   *callback* will be called exactly once per call to :meth:`call_later`.
+   If two callbacks are scheduled for exactly the same time, it is
+   undefined which will be called first.
+
+   The optional positional *args* will be passed to the callback when it
+   is called. If you want the callback to be called with some named
+   arguments, use a closure or :func:`functools.partial`.
+
+.. method:: BaseEventLoop.call_at(when, callback, *args)
+
+   Arrange for the *callback* to be called at the given absolute timestamp
+   *when* (an int or float), using the same time reference as :meth:`time`.
+
+   This method's behavior is the same as :meth:`call_later`.
+
+.. method:: BaseEventLoop.time()
+
+   Return the current time, as a :class:`float` value, according to the
+   event loop's internal clock.
+
+
+Creating connections
+^^^^^^^^^^^^^^^^^^^^
+
+.. method:: BaseEventLoop.create_connection(protocol_factory, host=None, port=None, \*, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None)
+
+   Create a streaming transport connection to a given Internet *host* and
+   *port*.  *protocol_factory* must be a callable returning a
+   :ref:`protocol <protocol>` instance.
+
+   This method returns a :ref:`coroutine <coroutine>` which will try to
+   establish the connection in the background.  When successful, the
+   coroutine returns a ``(transport, protocol)`` pair.
+
+   The chronological synopsis of the underlying operation is as follows:
+
+   #. The connection is established, and a :ref:`transport <transport>`
+      is created to represent it.
+
+   #. *protocol_factory* is called without arguments and must return a
+      :ref:`protocol <protocol>` instance.
+
+   #. The protocol instance is tied to the transport, and its
+      :meth:`connection_made` method is called.
+
+   #. The coroutine returns successfully with the ``(transport, protocol)``
+      pair.
+
+   The created transport is an implementation-dependent bidirectional stream.
+
+   .. note::
+      *protocol_factory* can be any kind of callable, not necessarily
+      a class.  For example, if you want to use a pre-created
+      protocol instance, you can pass ``lambda: my_protocol``.
+
+   Options allowing to change how the connection is created:
+
+   * *ssl*: if given and not false, a SSL/TLS transport is created
+     (by default a plain TCP transport is created).  If *ssl* is
+     a :class:`ssl.SSLContext` object, this context is used to create
+     the transport; if *ssl* is :const:`True`, a context with some
+     unspecified default settings is used.
+
+   * *server_hostname*, is only for use together with *ssl*,
+     and sets or overrides the hostname that the target server's certificate
+     will be matched against.  By default the value of the *host* argument
+     is used.  If *host* is empty, there is no default and you must pass a
+     value for *server_hostname*.  If *server_hostname* is an empty
+     string, hostname matching is disabled (which is a serious security
+     risk, allowing for man-in-the-middle-attacks).
+
+   * *family*, *proto*, *flags* are the optional address family, protocol
+     and flags to be passed through to getaddrinfo() for *host* resolution.
+     If given, these should all be integers from the corresponding
+     :mod:`socket` module constants.
+
+   * *sock*, if given, should be an existing, already connected
+     :class:`socket.socket` object to be used by the transport.
+     If *sock* is given, none of *host*, *port*, *family*, *proto*, *flags*
+     and *local_addr* should be specified.
+
+   * *local_addr*, if given, is a ``(local_host, local_port)`` tuple used
+     to bind the socket to locally.  The *local_host* and *local_port*
+     are looked up using getaddrinfo(), similarly to *host* and *port*.
+
+
+Creating listening connections
+------------------------------
+
+.. method:: BaseEventLoop.create_server(protocol_factory, host=None, port=None, \*, family=socket.AF_UNSPEC, flags=socket.AI_PASSIVE, sock=None, backlog=100, ssl=None, reuse_address=None)
+
+   A :ref:`coroutine <coroutine>` which creates a TCP server bound to host and
+   port.
+
+   The return value is a :class:`AbstractServer` object which can be used to stop
+   the service.
+
+   If *host* is an empty string or None all interfaces are assumed
+   and a list of multiple sockets will be returned (most likely
+   one for IPv4 and another one for IPv6).
+
+   *family* can be set to either :data:`~socket.AF_INET` or
+   :data:`~socket.AF_INET6` to force the socket to use IPv4 or IPv6. If not set
+   it will be determined from host (defaults to :data:`~socket.AF_UNSPEC`).
+
+   *flags* is a bitmask for :meth:`getaddrinfo`.
+
+   *sock* can optionally be specified in order to use a preexisting
+   socket object.
+
+   *backlog* is the maximum number of queued connections passed to
+   :meth:`~socket.socket.listen` (defaults to 100).
+
+   ssl can be set to an :class:`~ssl.SSLContext` to enable SSL over the
+   accepted connections.
+
+   *reuse_address* tells the kernel to reuse a local socket in
+   TIME_WAIT state, without waiting for its natural timeout to
+   expire. If not specified will automatically be set to True on
+   UNIX.
+
+   This method returns a :ref:`coroutine <coroutine>`.
+
+.. method:: BaseEventLoop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, \*, family=0, proto=0, flags=0)
+
+   Create datagram connection.
+
+   This method returns a :ref:`coroutine <coroutine>`.
+
+
+
+Resolve name
+------------
+
+.. method:: BaseEventLoop.getaddrinfo(host, port, \*, family=0, type=0, proto=0, flags=0)
+
+   XXX
+
+.. method:: BaseEventLoop.getnameinfo(sockaddr, flags=0)
+
+   XXX
+
+
+Running subprocesses
+--------------------
+
+Run subprocesses asynchronously using the :mod:`subprocess` module.
+
+.. method:: BaseEventLoop.subprocess_exec(protocol_factory, \*args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=False, shell=False, bufsize=0, \*\*kwargs)
+
+   XXX
+
+   This method returns a :ref:`coroutine <coroutine>`.
+
+   See the constructor of the :class:`subprocess.Popen` class for parameters.
+
+.. method:: BaseEventLoop.subprocess_shell(protocol_factory, cmd, \*, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=False, shell=True, bufsize=0, \*\*kwargs)
+
+   XXX
+
+   This method returns a :ref:`coroutine <coroutine>`.
+
+   See the constructor of the :class:`subprocess.Popen` class for parameters.
+
+.. method:: BaseEventLoop.connect_read_pipe(protocol_factory, pipe)
+
+   Register read pipe in eventloop.
+
+   *protocol_factory* should instantiate object with :class:`Protocol`
+   interface.  pipe is file-like object already switched to nonblocking.
+   Return pair (transport, protocol), where transport support
+   :class:`ReadTransport` interface.
+
+   This method returns a :ref:`coroutine <coroutine>`.
+
+.. method:: BaseEventLoop.connect_write_pipe(protocol_factory, pipe)
+
+   Register write pipe in eventloop.
+
+   *protocol_factory* should instantiate object with :class:`BaseProtocol`
+   interface.  Pipe is file-like object already switched to nonblocking.
+   Return pair (transport, protocol), where transport support
+   :class:`WriteTransport` interface.
+
+   This method returns a :ref:`coroutine <coroutine>`.
+
+
+Executor
+--------
+
+Call a function in an :class:`~concurrent.futures.Executor` (pool of threads or
+pool of processes). By default, an event loop uses a thread pool executor
+(:class:`~concurrent.futures.ThreadPoolExecutor`).
+
+.. method:: BaseEventLoop.run_in_executor(executor, callback, \*args)
+
+   Arrange for a callback to be called in the specified executor.
+
+   *executor* is a :class:`~concurrent.futures.Executor` instance,
+   the default executor is used if *executor* is ``None``.
+
+.. method:: BaseEventLoop.set_default_executor(executor)
+
+   Set the default executor used by :meth:`run_in_executor`.
+
+
+Examples
+--------
+
+Hello World (callback)
+^^^^^^^^^^^^^^^^^^^^^^
+
+Print ``Hello World`` every two seconds, using a callback::
+
+    import asyncio
+
+    def print_and_repeat(loop):
+        print('Hello World')
+        loop.call_later(2, print_and_repeat, loop)
+
+    loop = asyncio.get_event_loop()
+    print_and_repeat(loop)
+    loop.run_forever()
+
+
+Hello World (coroutine)
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Print ``Hello World`` every two seconds, using a coroutine::
+
+    import asyncio
+
+    @asyncio.coroutine
+    def greet_every_two_seconds():
+        while True:
+            print('Hello World')
+            yield from asyncio.sleep(2)
+
+    loop = asyncio.get_event_loop()
+    loop.run_until_complete(greet_every_two_seconds())
+