18.5.1. Base Event Loop
***********************

**Source code:** Lib/asyncio/events.py

The event loop is the central execution device provided by "asyncio".
It provides multiple facilities, including:

* Registering, executing and cancelling delayed calls (timeouts).

* Creating client and server transports for various kinds of
  communication.

* Launching subprocesses and the associated transports for
  communication with an external program.

* Delegating costly function calls to a pool of threads.

class asyncio.BaseEventLoop

   This class is an implementation detail.  It is a subclass of
   "AbstractEventLoop" and may be a base class of concrete event loop
   implementations found in "asyncio".  It should not be used
   directly; use "AbstractEventLoop" instead. "BaseEventLoop" should
   not be subclassed by third-party code; the internal interface is
   not stable.

class asyncio.AbstractEventLoop

   Abstract base class of event loops.

   This class is not thread safe.


18.5.1.1. Run an event loop
===========================

AbstractEventLoop.run_forever()

   Run until "stop()" is called.  If "stop()" is called before
   "run_forever()" is called, this polls the I/O selector once with a
   timeout of zero, runs all callbacks scheduled in response to I/O
   events (and those that were already scheduled), and then exits. If
   "stop()" is called while "run_forever()" is running, this will run
   the current batch of callbacks and then exit.  Note that callbacks
   scheduled by callbacks will not run in that case; they will run the
   next time "run_forever()" is called.

   Changed in version 3.5.1.

AbstractEventLoop.run_until_complete(future)

   Run until the "Future" is done.

   If the argument is a coroutine object, it is wrapped by
   "ensure_future()".

   Return the Future’s result, or raise its exception.

AbstractEventLoop.is_running()

   Returns running status of event loop.

AbstractEventLoop.stop()

   Stop running the event loop.

   This causes "run_forever()" to exit at the next suitable
   opportunity (see there for more details).

   Changed in version 3.5.1.

AbstractEventLoop.is_closed()

   Returns "True" if the event loop was closed.

   New in version 3.4.2.

AbstractEventLoop.close()

   Close the event loop. The loop must not be running.  Pending
   callbacks will be lost.

   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.

coroutine AbstractEventLoop.shutdown_asyncgens()

   Schedule all currently open *asynchronous generator* objects to
   close with an "aclose()" call.  After calling this method, the
   event loop will issue a warning whenever a new asynchronous
   generator is iterated.  Should be used to finalize all scheduled
   asynchronous generators reliably.  Example:

      try:
          loop.run_forever()
      finally:
          loop.run_until_complete(loop.shutdown_asyncgens())
          loop.close()

   New in version 3.6.


18.5.1.2. Calls
===============

Most "asyncio" functions don’t accept keywords. If you want to pass
keywords to your callback, use "functools.partial()". For example,
"loop.call_soon(functools.partial(print, "Hello", flush=True))" will
call "print("Hello", flush=True)".

Note: "functools.partial()" is better than "lambda" functions,
  because "asyncio" can inspect "functools.partial()" object to
  display parameters in debug mode, whereas "lambda" functions have a
  poor representation.

AbstractEventLoop.call_soon(callback, *args)

   Arrange for a callback to be called as soon as possible.  The
   callback is called after "call_soon()" returns, when control
   returns to the event loop.

   This operates as a FIFO (first-in, first-out) 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.

   An instance of "asyncio.Handle" is returned, which can be used to
   cancel the callback.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.call_soon_threadsafe(callback, *args)

   Like "call_soon()", but thread safe.

   See the concurrency and multithreading section of the
   documentation.


18.5.1.3. 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 "time.time()".

Note: Timeouts (relative *delay* or absolute *when*) should not
  exceed one day.

AbstractEventLoop.call_later(delay, callback, *args)

   Arrange for the *callback* to be called after the given *delay*
   seconds (either an int or float).

   An instance of "asyncio.Handle" is returned, which can be used to
   cancel the callback.

   *callback* will be called exactly once per call to "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 "functools.partial()".

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.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 "AbstractEventLoop.time()".

   This method’s behavior is the same as "call_later()".

   An instance of "asyncio.Handle" is returned, which can be used to
   cancel the callback.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.time()

   Return the current time, as a "float" value, according to the event
   loop’s internal clock.

See also: The "asyncio.sleep()" function.


18.5.1.4. Futures
=================

AbstractEventLoop.create_future()

   Create an "asyncio.Future" object attached to the loop.

   This is a preferred way to create futures in asyncio, as event loop
   implementations can provide alternative implementations of the
   Future class (with better performance or instrumentation).

   New in version 3.5.2.


18.5.1.5. Tasks
===============

AbstractEventLoop.create_task(coro)

   Schedule the execution of a coroutine object: wrap it in a future.
   Return a "Task" object.

   Third-party event loops can use their own subclass of "Task" for
   interoperability. In this case, the result type is a subclass of
   "Task".

   This method was added in Python 3.4.2. Use the "async()" function
   to support also older Python versions.

   New in version 3.4.2.

AbstractEventLoop.set_task_factory(factory)

   Set a task factory that will be used by
   "AbstractEventLoop.create_task()".

   If *factory* is "None" the default task factory will be set.

   If *factory* is a *callable*, it should have a signature matching
   "(loop, coro)", where *loop* will be a reference to the active
   event loop, *coro* will be a coroutine object.  The callable must
   return an "asyncio.Future" compatible object.

   New in version 3.4.4.

AbstractEventLoop.get_task_factory()

   Return a task factory, or "None" if the default one is in use.

   New in version 3.4.4.


18.5.1.6. Creating connections
==============================

coroutine AbstractEventLoop.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*: socket family "AF_INET" or "AF_INET6" depending on
   *host* (or *family* if specified), socket type "SOCK_STREAM".
   *protocol_factory* must be a callable returning a protocol
   instance.

   This method is a 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:

   1. The connection is established, and a transport is created to
      represent it.

   2. *protocol_factory* is called without arguments and must
      return a protocol instance.

   3. The protocol instance is tied to the transport, and its
      "connection_made()" method is called.

   4. 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 that 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
     "ssl.SSLContext" object, this context is used to create the
     transport; if *ssl* is "True", a context with some unspecified
     default settings is used.

     See also: SSL/TLS security considerations

   * *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 "socket" module constants.

   * *sock*, if given, should be an existing, already connected
     "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*.

   Changed in version 3.5: On Windows with "ProactorEventLoop",
   SSL/TLS is now supported.

   See also: The "open_connection()" function can be used to get a
     pair of ("StreamReader", "StreamWriter") instead of a protocol.

coroutine AbstractEventLoop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, *, family=0, proto=0, flags=0, reuse_address=None, reuse_port=None, allow_broadcast=None, sock=None)

   Create datagram connection: socket family "AF_INET" or "AF_INET6"
   depending on *host* (or *family* if specified), socket type
   "SOCK_DGRAM". *protocol_factory* must be a callable returning a
   protocol instance.

   This method is a coroutine which will try to establish the
   connection in the background.  When successful, the coroutine
   returns a "(transport, protocol)" pair.

   Options changing how the connection is created:

   * *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()".

   * *remote_addr*, if given, is a "(remote_host, remote_port)"
     tuple used to connect the socket to a remote address.  The
     *remote_host* and *remote_port* are looked up using
     "getaddrinfo()".

   * *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 "socket" module constants.

   * *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.

   * *reuse_port* tells the kernel to allow this endpoint to be
     bound to the same port as other existing endpoints are bound to,
     so long as they all set this flag when being created. This option
     is not supported on Windows and some UNIX’s. If the
     "SO_REUSEPORT" constant is not defined then this capability is
     unsupported.

   * *allow_broadcast* tells the kernel to allow this endpoint to
     send messages to the broadcast address.

   * *sock* can optionally be specified in order to use a
     preexisting, already connected, "socket.socket" object to be used
     by the transport. If specified, *local_addr* and *remote_addr*
     should be omitted (must be "None").

   On Windows with "ProactorEventLoop", this method is not supported.

   See UDP echo client protocol and UDP echo server protocol examples.

   Changed in version 3.4.4: The *family*, *proto*, *flags*,
   *reuse_address*, *reuse_port, *allow_broadcast*, and *sock*
   parameters were added.

coroutine AbstractEventLoop.create_unix_connection(protocol_factory, path, *, ssl=None, sock=None, server_hostname=None)

   Create UNIX connection: socket family "AF_UNIX", socket type
   "SOCK_STREAM". The "AF_UNIX" socket family is used to communicate
   between processes on the same machine efficiently.

   This method is a coroutine which will try to establish the
   connection in the background.  When successful, the coroutine
   returns a "(transport, protocol)" pair.

   *path* is the name of a UNIX domain socket, and is required unless
   a *sock* parameter is specified.  Abstract UNIX sockets, "str", and
   "bytes" paths are supported.

   See the "AbstractEventLoop.create_connection()" method for
   parameters.

   Availability: UNIX.


18.5.1.7. Creating listening connections
========================================

coroutine AbstractEventLoop.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, reuse_port=None)

   Create a TCP server (socket type "SOCK_STREAM") bound to *host* and
   *port*.

   Return a "Server" object, its "sockets" attribute contains created
   sockets. Use the "Server.close()" method to stop the server: close
   listening sockets.

   Parameters:

   * The *host* parameter can be a string, in that case the TCP
     server is bound to *host* and *port*. The *host* parameter can
     also be a sequence of strings and in that case the TCP server is
     bound to all hosts of the sequence. 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 "socket.AF_INET" or "AF_INET6" to
     force the socket to use IPv4 or IPv6. If not set it will be
     determined from host (defaults to "socket.AF_UNSPEC").

   * *flags* is a bitmask for "getaddrinfo()".

   * *sock* can optionally be specified in order to use a
     preexisting socket object. If specified, *host* and *port* should
     be omitted (must be "None").

   * *backlog* is the maximum number of queued connections passed to
     "listen()" (defaults to 100).

   * *ssl* can be set to an "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.

   * *reuse_port* tells the kernel to allow this endpoint to be
     bound to the same port as other existing endpoints are bound to,
     so long as they all set this flag when being created. This option
     is not supported on Windows.

   This method is a coroutine.

   Changed in version 3.5: On Windows with "ProactorEventLoop",
   SSL/TLS is now supported.

   See also: The function "start_server()" creates a
     ("StreamReader", "StreamWriter") pair and calls back a function
     with this pair.

   Changed in version 3.5.1: The *host* parameter can now be a
   sequence of strings.

coroutine AbstractEventLoop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None)

   Similar to "AbstractEventLoop.create_server()", but specific to the
   socket family "AF_UNIX".

   This method is a coroutine.

   Availability: UNIX.

coroutine BaseEventLoop.connect_accepted_socket(protocol_factory, sock, *, ssl=None)

   Handle an accepted connection.

   This is used by servers that accept connections outside of asyncio
   but that use asyncio to handle them.

   Parameters:

   * *sock* is a preexisting socket object returned from an "accept"
     call.

   * *ssl* can be set to an "SSLContext" to enable SSL over the
     accepted connections.

   This method is a coroutine.  When completed, the coroutine returns
   a "(transport, protocol)" pair.

   New in version 3.5.3.


18.5.1.8. Watch file descriptors
================================

On Windows with "SelectorEventLoop", only socket handles are supported
(ex: pipe file descriptors are not supported).

On Windows with "ProactorEventLoop", these methods are not supported.

AbstractEventLoop.add_reader(fd, callback, *args)

   Start watching the file descriptor for read availability and then
   call the *callback* with specified arguments.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_reader(fd)

   Stop watching the file descriptor for read availability.

AbstractEventLoop.add_writer(fd, callback, *args)

   Start watching the file descriptor for write availability and then
   call the *callback* with specified arguments.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_writer(fd)

   Stop watching the file descriptor for write availability.

The watch a file descriptor for read events example uses the low-level
"AbstractEventLoop.add_reader()" method to register the file
descriptor of a socket.


18.5.1.9. Low-level socket operations
=====================================

coroutine AbstractEventLoop.sock_recv(sock, nbytes)

   Receive data from the socket.  Modeled after blocking
   "socket.socket.recv()" method.

   The return value is a bytes object representing the data received.
   The maximum amount of data to be received at once is specified by
   *nbytes*.

   With "SelectorEventLoop" event loop, the socket *sock* must be non-
   blocking.

   This method is a coroutine.

coroutine AbstractEventLoop.sock_sendall(sock, data)

   Send data to the socket.  Modeled after blocking
   "socket.socket.sendall()" method.

   The socket must be connected to a remote socket. This method
   continues to send data from *data* until either all data has been
   sent or an error occurs.  "None" is returned on success.  On error,
   an exception is raised, and there is no way to determine how much
   data, if any, was successfully processed by the receiving end of
   the connection.

   With "SelectorEventLoop" event loop, the socket *sock* must be non-
   blocking.

   This method is a coroutine.

coroutine AbstractEventLoop.sock_connect(sock, address)

   Connect to a remote socket at *address*.  Modeled after blocking
   "socket.socket.connect()" method.

   With "SelectorEventLoop" event loop, the socket *sock* must be non-
   blocking.

   This method is a coroutine.

   Changed in version 3.5.2: "address" no longer needs to be resolved.
   "sock_connect" will try to check if the *address* is already
   resolved by calling "socket.inet_pton()".  If not,
   "AbstractEventLoop.getaddrinfo()" will be used to resolve the
   *address*.

   See also: "AbstractEventLoop.create_connection()" and
     "asyncio.open_connection()".

coroutine AbstractEventLoop.sock_accept(sock)

   Accept a connection.  Modeled after blocking
   "socket.socket.accept()".

   The socket must be bound to an address and listening for
   connections. The return value is a pair "(conn, address)" where
   *conn* is a *new* socket object usable to send and receive data on
   the connection, and *address* is the address bound to the socket on
   the other end of the connection.

   The socket *sock* must be non-blocking.

   This method is a coroutine.

   See also: "AbstractEventLoop.create_server()" and
     "start_server()".


18.5.1.10. Resolve host name
============================

coroutine AbstractEventLoop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)

   This method is a coroutine, similar to "socket.getaddrinfo()"
   function but non-blocking.

coroutine AbstractEventLoop.getnameinfo(sockaddr, flags=0)

   This method is a coroutine, similar to "socket.getnameinfo()"
   function but non-blocking.


18.5.1.11. Connect pipes
========================

On Windows with "SelectorEventLoop", these methods are not supported.
Use "ProactorEventLoop" to support pipes on Windows.

coroutine AbstractEventLoop.connect_read_pipe(protocol_factory, pipe)

   Register read pipe in eventloop.

   *protocol_factory* should instantiate object with "Protocol"
   interface.  *pipe* is a *file-like object*. Return pair
   "(transport, protocol)", where *transport* supports the
   "ReadTransport" interface.

   With "SelectorEventLoop" event loop, the *pipe* is set to non-
   blocking mode.

   This method is a coroutine.

coroutine AbstractEventLoop.connect_write_pipe(protocol_factory, pipe)

   Register write pipe in eventloop.

   *protocol_factory* should instantiate object with "BaseProtocol"
   interface. *pipe* is *file-like object*. Return pair "(transport,
   protocol)", where *transport* supports "WriteTransport" interface.

   With "SelectorEventLoop" event loop, the *pipe* is set to non-
   blocking mode.

   This method is a coroutine.

See also: The "AbstractEventLoop.subprocess_exec()" and
  "AbstractEventLoop.subprocess_shell()" methods.


18.5.1.12. UNIX signals
=======================

Availability: UNIX only.

AbstractEventLoop.add_signal_handler(signum, callback, *args)

   Add a handler for a signal.

   Raise "ValueError" if the signal number is invalid or uncatchable.
   Raise "RuntimeError" if there is a problem setting up the handler.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_signal_handler(sig)

   Remove a handler for a signal.

   Return "True" if a signal handler was removed, "False" if not.

See also: The "signal" module.


18.5.1.13. Executor
===================

Call a function in an "Executor" (pool of threads or pool of
processes). By default, an event loop uses a thread pool executor
("ThreadPoolExecutor").

coroutine AbstractEventLoop.run_in_executor(executor, func, *args)

   Arrange for a *func* to be called in the specified executor.

   The *executor* argument should be an "Executor" instance. The
   default executor is used if *executor* is "None".

   Use functools.partial to pass keywords to the *func*.

   This method is a coroutine.

   Changed in version 3.5.3: "BaseEventLoop.run_in_executor()" no
   longer configures the "max_workers" of the thread pool executor it
   creates, instead leaving it up to the thread pool executor
   ("ThreadPoolExecutor") to set the default.

AbstractEventLoop.set_default_executor(executor)

   Set the default executor used by "run_in_executor()".


18.5.1.14. Error Handling API
=============================

Allows customizing how exceptions are handled in the event loop.

AbstractEventLoop.set_exception_handler(handler)

   Set *handler* as the new event loop exception handler.

   If *handler* is "None", the default exception handler will be set.

   If *handler* is a callable object, it should have a matching
   signature to "(loop, context)", where "loop" will be a reference to
   the active event loop, "context" will be a "dict" object (see
   "call_exception_handler()" documentation for details about
   context).

AbstractEventLoop.get_exception_handler()

   Return the exception handler, or "None" if the default one is in
   use.

   New in version 3.5.2.

AbstractEventLoop.default_exception_handler(context)

   Default exception handler.

   This is called when an exception occurs and no exception handler is
   set, and can be called by a custom exception handler that wants to
   defer to the default behavior.

   *context* parameter has the same meaning as in
   "call_exception_handler()".

AbstractEventLoop.call_exception_handler(context)

   Call the current event loop exception handler.

   *context* is a "dict" object containing the following keys (new
   keys may be introduced later):

   * ‘message’: Error message;

   * ‘exception’ (optional): Exception object;

   * ‘future’ (optional): "asyncio.Future" instance;

   * ‘handle’ (optional): "asyncio.Handle" instance;

   * ‘protocol’ (optional): Protocol instance;

   * ‘transport’ (optional): Transport instance;

   * ‘socket’ (optional): "socket.socket" instance.

   Note: Note: this method should not be overloaded in subclassed
     event loops.  For any custom exception handling, use
     "set_exception_handler()" method.


18.5.1.15. Debug mode
=====================

AbstractEventLoop.get_debug()

   Get the debug mode ("bool") of the event loop.

   The default value is "True" if the environment variable
   "PYTHONASYNCIODEBUG" is set to a non-empty string, "False"
   otherwise.

   New in version 3.4.2.

AbstractEventLoop.set_debug(enabled: bool)

   Set the debug mode of the event loop.

   New in version 3.4.2.

See also: The debug mode of asyncio.


18.5.1.16. Server
=================

class asyncio.Server

   Server listening on sockets.

   Object created by the "AbstractEventLoop.create_server()" method
   and the "start_server()" function. Don’t instantiate the class
   directly.

   close()

      Stop serving: close listening sockets and set the "sockets"
      attribute to "None".

      The sockets that represent existing incoming client connections
      are left open.

      The server is closed asynchronously, use the "wait_closed()"
      coroutine to wait until the server is closed.

   coroutine wait_closed()

      Wait until the "close()" method completes.

      This method is a coroutine.

   sockets

      List of "socket.socket" objects the server is listening to, or
      "None" if the server is closed.


18.5.1.17. Handle
=================

class asyncio.Handle

   A callback wrapper object returned by
   "AbstractEventLoop.call_soon()",
   "AbstractEventLoop.call_soon_threadsafe()",
   "AbstractEventLoop.call_later()", and
   "AbstractEventLoop.call_at()".

   cancel()

      Cancel the call.  If the callback is already canceled or
      executed, this method has no effect.


18.5.1.18. Event loop examples
==============================


18.5.1.18.1. Hello World with call_soon()
-----------------------------------------

Example using the "AbstractEventLoop.call_soon()" method to schedule a
callback. The callback displays ""Hello World"" and then stops the
event loop:

   import asyncio

   def hello_world(loop):
       print('Hello World')
       loop.stop()

   loop = asyncio.get_event_loop()

   # Schedule a call to hello_world()
   loop.call_soon(hello_world, loop)

   # Blocking call interrupted by loop.stop()
   loop.run_forever()
   loop.close()

See also: The Hello World coroutine example uses a coroutine.


18.5.1.18.2. Display the current date with call_later()
-------------------------------------------------------

Example of callback displaying the current date every second. The
callback uses the "AbstractEventLoop.call_later()" method to
reschedule itself during 5 seconds, and then stops the event loop:

   import asyncio
   import datetime

   def display_date(end_time, loop):
       print(datetime.datetime.now())
       if (loop.time() + 1.0) < end_time:
           loop.call_later(1, display_date, end_time, loop)
       else:
           loop.stop()

   loop = asyncio.get_event_loop()

   # Schedule the first call to display_date()
   end_time = loop.time() + 5.0
   loop.call_soon(display_date, end_time, loop)

   # Blocking call interrupted by loop.stop()
   loop.run_forever()
   loop.close()

See also: The coroutine displaying the current date example uses a
  coroutine.


18.5.1.18.3. Watch a file descriptor for read events
----------------------------------------------------

Wait until a file descriptor received some data using the
"AbstractEventLoop.add_reader()" method and then close the event loop:

   import asyncio
   try:
       from socket import socketpair
   except ImportError:
       from asyncio.windows_utils import socketpair

   # Create a pair of connected file descriptors
   rsock, wsock = socketpair()
   loop = asyncio.get_event_loop()

   def reader():
       data = rsock.recv(100)
       print("Received:", data.decode())
       # We are done: unregister the file descriptor
       loop.remove_reader(rsock)
       # Stop the event loop
       loop.stop()

   # Register the file descriptor for read event
   loop.add_reader(rsock, reader)

   # Simulate the reception of data from the network
   loop.call_soon(wsock.send, 'abc'.encode())

   # Run the event loop
   loop.run_forever()

   # We are done, close sockets and the event loop
   rsock.close()
   wsock.close()
   loop.close()

See also: The register an open socket to wait for data using a
  protocol example uses a low-level protocol created by the
  "AbstractEventLoop.create_connection()" method.

  The register an open socket to wait for data using streams example
  uses high-level streams created by the "open_connection()" function
  in a coroutine.


18.5.1.18.4. Set signal handlers for SIGINT and SIGTERM
-------------------------------------------------------

Register handlers for signals "SIGINT" and "SIGTERM" using the
"AbstractEventLoop.add_signal_handler()" method:

   import asyncio
   import functools
   import os
   import signal

   def ask_exit(signame):
       print("got signal %s: exit" % signame)
       loop.stop()

   loop = asyncio.get_event_loop()
   for signame in ('SIGINT', 'SIGTERM'):
       loop.add_signal_handler(getattr(signal, signame),
                               functools.partial(ask_exit, signame))

   print("Event loop running forever, press Ctrl+C to interrupt.")
   print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
   try:
       loop.run_forever()
   finally:
       loop.close()

This example only works on UNIX.
