18.5.5. Streams (coroutine based API)
*************************************

**Source code:** Lib/asyncio/streams.py


18.5.5.1. Stream functions
==========================

Note: The top-level functions in this module are meant as
  convenience wrappers only; there’s really nothing special there, and
  if they don’t do exactly what you want, feel free to copy their
  code.

coroutine 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
   "AbstractEventLoop.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").

   This function is a coroutine.

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 converted into a "Task".

   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.

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.

coroutine 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)

   This class is not thread safe.

   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.

   coroutine 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.

   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.

   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.

   coroutine readuntil(separator=b'\n')

      Read data from the stream until "separator" is found.

      On success, the data and separator will be removed from the
      internal buffer (consumed). Returned data will include the
      separator at the end.

      Configured stream limit is used to check result. Limit sets the
      maximal length of data that can be returned, not counting the
      separator.

      If an EOF occurs and the complete separator is still not found,
      an "IncompleteReadError" exception will be raised, and the
      internal buffer will be reset.  The
      "IncompleteReadError.partial" attribute may contain the
      separator partially.

      If the data cannot be read because of over limit, a
      "LimitOverrunError" exception  will be raised, and the data will
      be left in the internal buffer, so it can be read again.

      New in version 3.5.2.

   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.

   This class is not thread safe.

   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()".

   coroutine drain()

      Let the write buffer of the underlying transport a chance to be
      flushed.

      The intended use is to write:

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

      When the size of the transport buffer reaches the high-water
      limit (the protocol is paused), block until the size of the
      buffer is drained down to the low-water limit and the protocol
      is resumed. When there is nothing to wait for, the yield-from
      continues immediately.

      Yielding from "drain()" gives the opportunity for the loop to
      schedule the write operation and flush the buffer. It should
      especially be used when a possibly large amount of data is
      written to the transport, and the coroutine does not yield-from
      between calls to "write()".

      This method is a coroutine.

   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". Subclass 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" from
   accidentally calling 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. LimitOverrunError
===========================

exception asyncio.LimitOverrunError

   Reached the buffer limit while looking for a separator.

   consumed

      Total number of to be consumed bytes.


18.5.5.7. Stream examples
=========================


18.5.5.7.1. TCP echo client using streams
-----------------------------------------

TCP echo client using the "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()

See also: The TCP echo client protocol example uses the
  "AbstractEventLoop.create_connection()" method.


18.5.5.7.2. TCP echo server using streams
-----------------------------------------

TCP echo server using the "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()

See also: The TCP echo server protocol example uses the
  "AbstractEventLoop.create_server()" method.


18.5.5.7.3. Get HTTP headers
----------------------------

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.ensure_future(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


18.5.5.7.4. Register an open socket to wait for data using streams
------------------------------------------------------------------

Coroutine waiting until a socket receives data using the
"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()

See also: The register an open socket to wait for data using a
  protocol example uses a low-level protocol created by the
  "AbstractEventLoop.create_connection()" method.

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