.. currentmodule:: asyncio

.. function:: open_connection(host=None, port=None, \*, loop=None, limit=None, **kwds)

   A wrapper for :meth:`~BaseEventLoop.create_connection()` returning a (reader,
   writer) pair.

   The reader returned is a :class:`StreamReader` instance; the writer is
   a :class:`StreamWriter` instance.

   The arguments are all the usual arguments to
   :meth:`BaseEventLoop.create_connection` except *protocol_factory*; most
   common are positional host and port, with various optional keyword arguments
   following.

   Additional optional keyword arguments are *loop* (to set the event loop
   instance to use) and *limit* (to set the buffer limit passed to the
   :class:`StreamReader`).

   (If you want to customize the :class:`StreamReader` and/or
   :class:`StreamReaderProtocol` classes, just copy the code -- there's really
   nothing special here except some convenience.)

   This function is a :ref:`coroutine <coroutine>`.

.. function:: start_server(client_connected_cb, host=None, port=None, \*, loop=None, limit=None, **kwds)

   Start a socket server, with a callback for each client connected. The return
   value is the same as :meth:`~BaseEventLoop.create_server()`.

   The *client_connected_cb* parameter is called with two parameters:
   *client_reader*, *client_writer*.  *client_reader* is a
   :class:`StreamReader` object, while *client_writer* is a
   :class:`StreamWriter` object.  The *client_connected_cb* parameter can
   either be a plain callback function or a :ref:`coroutine function
   <coroutine>`; if it is a coroutine function, it will be automatically
   converted into a :class:`Task`.

   The rest of the arguments are all the usual arguments to
   :meth:`~BaseEventLoop.create_server()` except *protocol_factory*; most
   common are positional *host* and *port*, with various optional keyword
   arguments following.

   Additional optional keyword arguments are *loop* (to set the event loop
   instance to use) and *limit* (to set the buffer limit passed to the
   :class:`StreamReader`).

   This function is a :ref:`coroutine <coroutine>`.

.. function:: open_unix_connection(path=None, \*, loop=None, limit=None, **kwds)

   A wrapper for :meth:`~BaseEventLoop.create_unix_connection()` returning
   a (reader, writer) pair.

   See :func:`open_connection` for information about return value and other
   details.

   This function is a :ref:`coroutine <coroutine>`.

   Availability: UNIX.

.. function:: start_unix_server(client_connected_cb, path=None, \*, loop=None, limit=None, **kwds)

   Start a UNIX Domain Socket server, with a callback for each client connected.

   See :func:`start_server` for information about return value and other
   details.

   This function is a :ref:`coroutine <coroutine>`.

   Availability: UNIX.

.. method:: exception()

   Get the exception.

.. method:: feed_eof()

   Acknowledge the EOF.

.. method:: feed_data(data)

   Feed *data* bytes in the internal buffer.  Any operations waiting
   for the data will be resumed.

.. method:: set_exception(exc)

   Set the exception.

.. method:: set_transport(transport)

   Set the transport.

.. method:: read(n=-1)

   Read up to *n* bytes.  If *n* is not provided, or set to ``-1``,
   read until EOF and return all read bytes.

   If the EOF was received and the internal buffer is empty,
   return an empty ``bytes`` object.

   This method is a :ref:`coroutine <coroutine>`.

.. method:: readline()

   Read one line, where "line" is a sequence of bytes ending with ``\n``.

   If EOF is received, and ``\n`` was not found, the method will
   return the partial read bytes.

   If the EOF was received and the internal buffer is empty,
   return an empty ``bytes`` object.

   This method is a :ref:`coroutine <coroutine>`.

.. method:: readexactly(n)

   Read exactly *n* bytes. Raise an :exc:`IncompleteReadError` if the end of
   the stream is reached before *n* can be read, the
   :attr:`IncompleteReadError.partial` attribute of the exception contains
   the partial read bytes.

   This method is a :ref:`coroutine <coroutine>`.

.. method:: at_eof()

   Return ``True`` if the buffer is empty and :meth:`feed_eof` was called.

Wraps a Transport.

This exposes :meth:`write`, :meth:`writelines`, :meth:`can_write_eof()`, :meth:`write_eof`, :meth:`get_extra_info` and :meth:`close`. It adds :meth:`drain` which returns an optional :class:`Future` on which you can wait for flow control. It also adds a transport attribute which references the :class:`Transport` directly.

.. attribute:: transport

   Transport.

.. method:: can_write_eof()

   Return :const:`True` if the transport supports :meth:`write_eof`,
   :const:`False` if not. See :meth:`WriteTransport.can_write_eof`.

.. method:: close()

   Close the transport: see :meth:`BaseTransport.close`.

.. method:: drain()

   Wait until the write buffer of the underlying transport is flushed.

   The intended use is to write::

       w.write(data)
       yield from w.drain()

   When the transport buffer is full (the protocol is paused), block until
   the buffer is (partially) drained and the protocol is resumed. When there
   is nothing to wait for, the yield-from continues immediately.

   This method is a :ref:`coroutine <coroutine>`.

.. method:: get_extra_info(name, default=None)

   Return optional transport information: see
   :meth:`BaseTransport.get_extra_info`.

.. method:: write(data)

   Write some *data* bytes to the transport: see
   :meth:`WriteTransport.write`.

.. method:: writelines(data)

   Write a list (or any iterable) of data bytes to the transport:
   see :meth:`WriteTransport.writelines`.

.. method:: write_eof()

   Close the write end of the transport after flushing buffered data:
   see :meth:`WriteTransport.write_eof`.

Trivial helper class to adapt between :class:`Protocol` and :class:`StreamReader`. Sublclass of :class:`Protocol`.

stream_reader is a :class:`StreamReader` instance, client_connected_cb is an optional function called with (stream_reader, stream_writer) when a connection is made, loop is the event loop instance to use.

(This is a helper class instead of making :class:`StreamReader` itself a :class:`Protocol` subclass, because the :class:`StreamReader` has other potential uses, and to prevent the user of the :class:`StreamReader` to accidentally call inappropriate methods of the protocol.)

.. exception:: IncompleteReadError

    Incomplete read error, subclass of :exc:`EOFError`.

   .. attribute:: expected

      Total number of expected bytes (:class:`int`).

   .. attribute:: partial

      Read bytes string before the end of stream was reached (:class:`bytes`).

TCP echo client using the :func:`asyncio.open_connection` function:

import asyncio

@asyncio.coroutine
def tcp_echo_client(message, loop):
    reader, writer = yield from asyncio.open_connection('127.0.0.1', 8888,
                                                        loop=loop)

    print('Send: %r' % message)
    writer.write(message.encode())

    data = yield from reader.read(100)
    print('Received: %r' % data.decode())

    print('Close the socket')
    writer.close()

message = 'Hello World!'
loop = asyncio.get_event_loop()
loop.run_until_complete(tcp_echo_client(message, loop))
loop.close()

.. seealso::

   The :ref:`TCP echo client protocol <asyncio-tcp-echo-client-protocol>`
   example uses the :meth:`BaseEventLoop.create_connection` method.

TCP echo server using the :func:`asyncio.start_server` function:

import asyncio

@asyncio.coroutine
def handle_echo(reader, writer):
    data = yield from reader.read(100)
    message = data.decode()
    addr = writer.get_extra_info('peername')
    print("Received %r from %r" % (message, addr))

    print("Send: %r" % message)
    writer.write(data)
    yield from writer.drain()

    print("Close the client socket")
    writer.close()

loop = asyncio.get_event_loop()
coro = asyncio.start_server(handle_echo, '127.0.0.1', 8888, loop=loop)
server = loop.run_until_complete(coro)

# Serve requests until CTRL+c is pressed
print('Serving on {}'.format(server.sockets[0].getsockname()))
try:
    loop.run_forever()
except KeyboardInterrupt:
    pass

# Close the server
server.close()
loop.run_until_complete(server.wait_closed())
loop.close()

.. seealso::

   The :ref:`TCP echo server protocol <asyncio-tcp-echo-server-protocol>`
   example uses the :meth:`BaseEventLoop.create_server` method.

Simple example querying HTTP headers of the URL passed on the command line:

import asyncio
import urllib.parse
import sys

@asyncio.coroutine
def print_http_headers(url):
    url = urllib.parse.urlsplit(url)
    if url.scheme == 'https':
        connect = asyncio.open_connection(url.hostname, 443, ssl=True)
    else:
        connect = asyncio.open_connection(url.hostname, 80)
    reader, writer = yield from connect
    query = ('HEAD {path} HTTP/1.0\r\n'
             'Host: {hostname}\r\n'
             '\r\n').format(path=url.path or '/', hostname=url.hostname)
    writer.write(query.encode('latin-1'))
    while True:
        line = yield from reader.readline()
        if not line:
            break
        line = line.decode('latin1').rstrip()
        if line:
            print('HTTP header> %s' % line)

    # Ignore the body, close the socket
    writer.close()

url = sys.argv[1]
loop = asyncio.get_event_loop()
task = asyncio.async(print_http_headers(url))
loop.run_until_complete(task)
loop.close()

Usage:

python example.py http://example.com/path/page.html

or with HTTPS:

python example.py https://example.com/path/page.html

Coroutine waiting until a socket receives data using the :func:`open_connection` function:

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

@asyncio.coroutine
def wait_for_data(loop):
    # Create a pair of connected sockets
    rsock, wsock = socketpair()

    # Register the open socket to wait for data
    reader, writer = yield from asyncio.open_connection(sock=rsock, loop=loop)

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

    # Wait for data
    data = yield from reader.read(100)

    # Got data, we are done: close the socket
    print("Received:", data.decode())
    writer.close()

    # Close the second socket
    wsock.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(wait_for_data(loop))
loop.close()

.. seealso::

   The :ref:`register an open socket to wait for data using a protocol
   <asyncio-register-socket>` example uses a low-level protocol created by the
   :meth:`BaseEventLoop.create_connection` method.

   The :ref:`watch a file descriptor for read events
   <asyncio-watch-read-event>` example uses the low-level
   :meth:`BaseEventLoop.add_reader` method to register the file descriptor of a
   socket.