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# 18.5.5. Streams (high-level API)

## 18.5.5.1. Stream functions

`asyncio.``open_connection`(host=None, port=None, *, loop=None, limit=None, **kwds)

  A wrapper for `create_connection()` returning a (reader, writer) pair.

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

  The arguments are all the usual arguments to `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 `StreamReader`).

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

  This function is a coroutine.

`asyncio.``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 `create_server()`.

  The client_connected_cb parameter is called with two parameters: client_reader, client_writer. client_reader is a `StreamReader` object, while client_writer is a `StreamWriter` object. The client_connected_cb parameter can either be a plain callback function or a coroutine function; if it is a coroutine function, it will be automatically wrapped in a future using the `BaseEventLoop.create_task()` method.

  The rest of the arguments are all the usual arguments to `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 `StreamReader`).

  This function is a coroutine.

`asyncio.``open_unix_connection`(path=None, *, loop=None, limit=None, **kwds)

  A wrapper for `create_unix_connection()` returning a (reader, writer) pair.

  See `open_connection()` for information about return value and other details.

  This function is a coroutine.

  Availability: UNIX.

`asyncio.``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 `start_server()` for information about return value and other details.

  This function is a coroutine.

  Availability: UNIX.

## 18.5.5.2. StreamReader

class `asyncio.``StreamReader`(limit=None, loop=None)
  `exception`()

    Get the exception.

  `feed_eof`()

    Acknowledge the EOF.

  `feed_data`(data)

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

  `set_exception`(exc)

    Set the exception.

  `set_transport`(transport)

    Set the transport.

  `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 coroutine.

  `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 coroutine.

  `readexactly`(n)

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

    This method is a coroutine.

  `at_eof`()

    Return `True` if the buffer is empty and `feed_eof()` was called.

## 18.5.5.3. StreamWriter

class `asyncio.``StreamWriter`(transport, protocol, reader, loop)

  Wraps a Transport.

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

  `transport`

    Transport.

  `can_write_eof`()

    Return `True` if the transport supports `write_eof()`, `False` if not. See `WriteTransport.can_write_eof()`.

  `close`()

    Close the transport: see `BaseTransport.close()`.

  `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, `drain()` returns `()`, and the yield-from continues immediately. When the transport buffer is full (the protocol is paused), `drain()` creates and returns a `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.

  `get_extra_info`(name, default=None)

    Return optional transport information: see `BaseTransport.get_extra_info()`.

  `write`(data)

    Write some data bytes to the transport: see `WriteTransport.write()`.

  `writelines`(data)

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

  `write_eof`()

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

## 18.5.5.4. StreamReaderProtocol

class `asyncio.``StreamReaderProtocol`(stream_reader, client_connected_cb=None, loop=None)

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

  stream_reader is a `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 `StreamReader` itself a `Protocol` subclass, because the `StreamReader` has other potential uses, and to prevent the user of the `StreamReader` to accidentally call inappropriate methods of the protocol.)

## 18.5.5.5. IncompleteReadError

exception `asyncio.``IncompleteReadError`

  > Incomplete read error, subclass of `EOFError`.

  `expected`

    Total number of expected bytes (`int`).

  `partial`

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

## 18.5.5.6. 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

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