cpython/Doc/library/asyncio-stream.rst

.. currentmodule:: asyncio

.. _asyncio-streams:

++++++++++++++++++++++++
Streams (high-level API)
++++++++++++++++++++++++

Stream functions
================

.. 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.


StreamReader
============

.. class:: StreamReader(limit=None, loop=None)

   .. 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.


StreamWriter
============

.. class:: StreamWriter(transport, protocol, reader, loop)

   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`.


StreamReaderProtocol
====================

.. class:: StreamReaderProtocol(stream_reader, client_connected_cb=None, loop=None)

    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.)


IncompleteReadError
===================

.. 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`).


Example
=======

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)
        reader, writer = yield from asyncio.open_connection(url.hostname, 80)
        query = ('HEAD {url.path} HTTP/1.0\r\n'
                 'Host: {url.hostname}\r\n'
                 '\r\n').format(url=url)
        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)

    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