cpython/Doc/library/asyncio-stream.rst

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.. 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 first parameter, *client_connected_cb*, takes two parameters:
*client_reader*, *client_writer*. *client_reader* is a
:class:`StreamReader` object, while *client_writer* is a
:class:`StreamWriter` object. This 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. The return value is the same as
:meth:`~BaseEventLoop.create_server()`.
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`).
The return value is the same as :meth:`~BaseEventLoop.create_server()`, i.e.
a :class:`AbstractServer` object which can be used to stop the service.
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.
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.. 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.
This method has an unusual return value. The intended use is to write::
w.write(data)
yield from w.drain()
When there's nothing to wait for, :meth:`drain()` returns ``()``, and the
yield-from continues immediately. When the transport buffer is full (the
protocol is paused), :meth:`drain` creates and returns a
:class:`Future` and the yield-from will block until
that Future is completed, which will happen when the buffer is
(partially) drained and the protocol is resumed.
.. 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)
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loop.close()
Usage::
python example.py http://example.com/path/page.html