
Base Event Loop
***************

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

* 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 class asyncio.BaseEventLoop

   Base class of event loops.

   This class is *not thread safe*.


Run an event loop
=================

BaseEventLoop.run_forever()

   Run until "stop()" is called.

BaseEventLoop.run_until_complete(future)

   Run until the "Future" is done.

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

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

BaseEventLoop.is_running()

   Returns running status of event loop.

BaseEventLoop.stop()

   Stop running the event loop.

   Every callback scheduled before "stop()" is called will run.
   Callbacks scheduled after "stop()" is called will not run. However,
   those callbacks will run if "run_forever()" is called again later.

BaseEventLoop.is_closed()

   Returns "True" if the event loop was closed.

   New in version 3.4.2.

BaseEventLoop.close()

   Close the event loop. The loop must 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
=====

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.

BaseEventLoop.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 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.

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

BaseEventLoop.call_soon_threadsafe(callback, *args)

   Like "call_soon()", but thread safe.

   See the *concurrency and multithreading* section of the
   documentation.


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.

BaseEventLoop.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.

   *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*.

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 "BaseEventLoop.time()".

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

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

BaseEventLoop.time()

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

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


Coroutines
==========

BaseEventLoop.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.


Creating connections
====================

coroutine 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*: 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 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
     "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*.

   On Windows with "ProactorEventLoop", SSL/TLS is not supported.

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

coroutine BaseEventLoop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, *, family=0, proto=0, flags=0)

   Create datagram connection: socket family "AF_INET" or "AF_INET6"
   depending on *host* (or *family* if specified), socket type
   "SOCK_DGRAM".

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

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

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

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

coroutine BaseEventLoop.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.

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

   Availability: UNIX.


Creating listening connections
==============================

coroutine 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)

   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:

   * 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.

   This method is a *coroutine*.

   On Windows with "ProactorEventLoop", SSL/TLS is not supported.

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

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

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

   This method is a *coroutine*.

   Availability: UNIX.


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.

BaseEventLoop.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*.

BaseEventLoop.remove_reader(fd)

   Stop watching the file descriptor for read availability.

BaseEventLoop.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*.

BaseEventLoop.remove_writer(fd)

   Stop watching the file descriptor for write availability.

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


Low-level socket operations
===========================

coroutine BaseEventLoop.sock_recv(sock, nbytes)

   Receive data from the socket.  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*.

   See also: The "socket.socket.recv()" method.

coroutine BaseEventLoop.sock_sendall(sock, data)

   Send data to the socket.  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*.

   See also: The "socket.socket.sendall()" method.

coroutine BaseEventLoop.sock_connect(sock, address)

   Connect to a remote socket at *address*.

   The *address* must be already resolved to avoid the trap of hanging
   the entire event loop when the address requires doing a DNS lookup.
   For example, it must be an IP address, not an hostname, for
   "AF_INET" and "AF_INET6" address families. Use "getaddrinfo()" to
   resolve the hostname asynchronously.

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

   This method is a *coroutine*.

   See also: The "BaseEventLoop.create_connection()" method, the
     "open_connection()" function and the "socket.socket.connect()"
     method.

coroutine BaseEventLoop.sock_accept(sock)

   Accept a connection. 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: The "BaseEventLoop.create_server()" method, the
     "start_server()" function and the "socket.socket.accept()"
     method.


Resolve host name
=================

coroutine BaseEventLoop.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 BaseEventLoop.getnameinfo(sockaddr, flags=0)

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


Connect pipes
=============

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

coroutine BaseEventLoop.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 BaseEventLoop.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 "BaseEventLoop.subprocess_exec()" and
  "BaseEventLoop.subprocess_shell()" methods.


UNIX signals
============

Availability: UNIX only.

BaseEventLoop.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*.

BaseEventLoop.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.


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 BaseEventLoop.run_in_executor(executor, callback, *args)

   Arrange for a callback 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 callback*.

   This method is a *coroutine*.

BaseEventLoop.set_default_executor(executor)

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


Error Handling API
==================

Allows to customize how exceptions are handled in the event loop.

BaseEventLoop.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).

BaseEventLoop.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()".

BaseEventLoop.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.


Debug mode
==========

BaseEventLoop.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.

BaseEventLoop.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*.


Server
======

class class asyncio.Server

   Server listening on sockets.

   Object created by the "BaseEventLoop.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 leaved open.

      The server is closed asynchonously, 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.


Handle
======

class class asyncio.Handle

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

   cancel()

      Cancel the call.


Event loop examples
===================


Hello World with call_soon()
----------------------------

Example using the "BaseEventLoop.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*.


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

Example of callback displaying the current date every second. The
callback uses the "BaseEventLoop.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*.


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

Wait until a file descriptor received some data using the
"BaseEventLoop.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
  "BaseEventLoop.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.


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

Register handlers for signals "SIGINT" and "SIGTERM" using the
"BaseEventLoop.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.
