cpython/Doc/library/asyncio-protocol.rst

.. module:: asyncio

++++++++++++++++++++++++
Transports and protocols
++++++++++++++++++++++++

.. _transport:

Transports
==========

Transports are classed provided by :mod:`asyncio` in order to abstract
various kinds of communication channels.  You generally won't instantiate
a transport yourself; instead, you will call a :class:`BaseEventLoop` method
which will create the transport and try to initiate the underlying
communication channel, calling you back when it succeeds.

Once the communication channel is established, a transport is always
paired with a :ref:`protocol <protocol>` instance.  The protocol can
then call the transport's methods for various purposes.

:mod:`asyncio` currently implements transports for TCP, UDP, SSL, and
subprocess pipes.  The methods available on a transport depend on
the transport's kind.


BaseTransport
-------------

.. class:: BaseTransport

   Base class for transports.

   .. method:: close(self)

      Close the transport.  If the transport has a buffer for outgoing
      data, buffered data will be flushed asynchronously.  No more data
      will be received.  After all buffered data is flushed, the
      protocol's :meth:`connection_lost` method will be called with
      :const:`None` as its argument.


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

      Return optional transport information.  *name* is a string representing
      the piece of transport-specific information to get, *default* is the
      value to return if the information doesn't exist.

      This method allows transport implementations to easily expose
      channel-specific information.

      * socket:

        - ``'peername'``: the remote address to which the socket is connected,
          result of :meth:`socket.socket.getpeername` (``None`` on error)
        - ``'socket'``: :class:`socket.socket` instance
        - ``'sockname'``: the socket's own address,
          result of :meth:`socket.socket.getsockname`

      * SSL socket:

        - ``'compression'``: the compression algorithm being used as a string,
          or ``None`` if the connection isn't compressed; result of
          :meth:`ssl.SSLSocket.compression`
        - ``'cipher'``: a three-value tuple containing the name of the cipher
          being used, the version of the SSL protocol that defines its use, and
          the number of secret bits being used; result of
          :meth:`ssl.SSLSocket.cipher`
        - ``'peercert'``: peer certificate; result of
          :meth:`ssl.SSLSocket.getpeercert`
        - ``'sslcontext'``: :class:`ssl.SSLContext` instance

      * pipe:

        - ``'pipe'``: pipe object

      * subprocess:

        - ``'subprocess'``: :class:`subprocess.Popen` instance


ReadTransport
-------------

.. class:: ReadTransport

   Interface for read-only transports.

   .. method:: pause_reading()

      Pause the receiving end of the transport.  No data will be passed to
      the protocol's :meth:`data_received` method until meth:`resume_reading`
      is called.

   .. method:: resume_reading()

      Resume the receiving end.  The protocol's :meth:`data_received` method
      will be called once again if some data is available for reading.


WriteTransport
--------------

.. class:: WriteTransport

   Interface for write-only transports.

   .. method:: abort()

      Close the transport immediately, without waiting for pending operations
      to complete.  Buffered data will be lost.  No more data will be received.
      The protocol's :meth:`connection_lost` method will eventually be
      called with :const:`None` as its argument.

   .. method:: can_write_eof()

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

   .. method:: get_write_buffer_size()

      Return the current size of the output buffer used by the transport.

   .. method:: set_write_buffer_limits(high=None, low=None)

      Set the *high*- and *low*-water limits for write flow control.

      These two values control when call the protocol's
      :meth:`pause_writing` and :meth:`resume_writing` methods are called.
      If specified, the low-water limit must be less than or equal to the
      high-water limit.  Neither *high* nor *low* can be negative.

      The defaults are implementation-specific.  If only the
      high-water limit is given, the low-water limit defaults to a
      implementation-specific value less than or equal to the
      high-water limit.  Setting *high* to zero forces *low* to zero as
      well, and causes :meth:`pause_writing` to be called whenever the
      buffer becomes non-empty.  Setting *low* to zero causes
      :meth:`resume_writing` to be called only once the buffer is empty.
      Use of zero for either limit is generally sub-optimal as it
      reduces opportunities for doing I/O and computation
      concurrently.

   .. method:: write(data)

      Write some *data* bytes to the transport.

      This method does not block; it buffers the data and arranges for it
      to be sent out asynchronously.

   .. method:: writelines(list_of_data)

      Write a list (or any iterable) of data bytes to the transport.
      This is functionally equivalent to calling :meth:`write` on each
      element yielded by the iterable, but may be implemented more efficiently.

   .. method:: write_eof()

      Close the write end of the transport after flushing buffered data.
      Data may still be received.

      This method can raise :exc:`NotImplementedError` if the transport
      (e.g. SSL) doesn't support half-closes.


DatagramTransport
-----------------

.. method:: DatagramTransport.sendto(data, addr=None)

   Send the *data* bytes to the remote peer given by *addr* (a
   transport-dependent target address).  If *addr* is :const:`None`, the
   data is sent to the target address given on transport creation.

   This method does not block; it buffers the data and arranges for it
   to be sent out asynchronously.

.. method:: DatagramTransport.abort()

   Close the transport immediately, without waiting for pending operations
   to complete.  Buffered data will be lost.  No more data will be received.
   The protocol's :meth:`connection_lost` method will eventually be
   called with :const:`None` as its argument.


BaseSubprocessTransport
-----------------------

.. class:: BaseSubprocessTransport

   .. method:: get_pid()

      Return the subprocess process id as an integer.

   .. method:: get_pipe_transport(fd)

      Return the transport for the communication pipe correspondong to the
      integer file descriptor *fd*.  The return value can be a readable or
      writable streaming transport, depending on the *fd*.  If *fd* doesn't
      correspond to a pipe belonging to this transport, :const:`None` is
      returned.

   .. method:: get_returncode()

      Return the subprocess returncode as an integer or :const:`None`
      if it hasn't returned, similarly to the
      :attr:`subprocess.Popen.returncode` attribute.

   .. method:: kill(self)

      Kill the subprocess, as in :meth:`subprocess.Popen.kill`

      On POSIX systems, the function sends SIGKILL to the subprocess.
      On Windows, this method is an alias for :meth:`terminate`.

   .. method:: send_signal(signal)

      Send the *signal* number to the subprocess, as in
      :meth:`subprocess.Popen.send_signal`.

   .. method:: terminate()

      Ask the subprocess to stop, as in :meth:`subprocess.Popen.terminate`.
      This method is an alias for the :meth:`close` method.

      On POSIX systems, this method sends SIGTERM to the subprocess.
      On Windows, the Windows API function TerminateProcess() is called to
      stop the subprocess.


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:`~concurrent.futures.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:: close()

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

   .. method:: drain()

      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:`~concurrent.futures.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:: can_write_eof()

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

   .. method:: write_eof()

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


StreamReader
------------

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

   .. method:: exception()

      Get the exception.

   .. method:: feed_eof()

      XXX

   .. method:: feed_data(data)

      XXX

   .. method:: set_exception(exc)

      Set the exception.

   .. method:: set_transport(transport)

      Set the transport.

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

      XXX

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

   .. method:: readline()

      XXX

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

   .. method:: readexactly(n)

      XXX

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



.. _protocol:

Protocols
=========

:mod:`asyncio` provides base classes that you can subclass to implement
your network protocols.  Those classes are used in conjunction with
:ref:`transports <transport>` (see below): the protocol parses incoming
data and asks for the writing of outgoing data, while the transport is
responsible for the actual I/O and buffering.

When subclassing a protocol class, it is recommended you override certain
methods.  Those methods are callbacks: they will be called by the transport
on certain events (for example when some data is received); you shouldn't
call them yourself, unless you are implementing a transport.

.. note::
   All callbacks have default implementations, which are empty.  Therefore,
   you only need to implement the callbacks for the events in which you
   are interested.


Protocol classes
----------------

.. class:: Protocol

   The base class for implementing streaming protocols (for use with
   e.g. TCP and SSL transports).

.. class:: DatagramProtocol

   The base class for implementing datagram protocols (for use with
   e.g. UDP transports).

.. class:: SubprocessProtocol

   The base class for implementing protocols communicating with child
   processes (through a set of unidirectional pipes).


Connection callbacks
--------------------

These callbacks may be called on :class:`Protocol` and
:class:`SubprocessProtocol` instances:

.. method:: BaseProtocol.connection_made(transport)

   Called when a connection is made.

   The *transport* argument is the transport representing the
   connection.  You are responsible for storing it somewhere
   (e.g. as an attribute) if you need to.

.. method:: BaseProtocol.connection_lost(exc)

   Called when the connection is lost or closed.

   The argument is either an exception object or :const:`None`.
   The latter means a regular EOF is received, or the connection was
   aborted or closed by this side of the connection.

:meth:`connection_made` and :meth:`connection_lost` are called exactly once
per successful connection.  All other callbacks will be called between those
two methods, which allows for easier resource management in your protocol
implementation.

The following callbacks may be called only on :class:`SubprocessProtocol`
instances:

.. method:: SubprocessProtocol.pipe_data_received(fd, data)

   Called when the child process writes data into its stdout or stderr pipe.
   *fd* is the integer file descriptor of the pipe.  *data* is a non-empty
   bytes object containing the data.

.. method:: SubprocessProtocol.pipe_connection_lost(fd, exc)

   Called when one of the pipes communicating with the child process
   is closed.  *fd* is the integer file descriptor that was closed.

.. method:: SubprocessProtocol.process_exited()

   Called when the child process has exited.


Streaming protocols
-------------------

The following callbacks are called on :class:`Protocol` instances:

.. method:: Protocol.data_received(data)

   Called when some data is received.  *data* is a non-empty bytes object
   containing the incoming data.

   .. note::
      Whether the data is buffered, chunked or reassembled depends on
      the transport.  In general, you shouldn't rely on specific semantics
      and instead make your parsing generic and flexible enough.  However,
      data is always received in the correct order.

.. method:: Protocol.eof_received()

   Calls when the other end signals it won't send any more data
   (for example by calling :meth:`write_eof`, if the other end also uses
   asyncio).

   This method may return a false value (including None), in which case
   the transport will close itself.  Conversely, if this method returns a
   true value, closing the transport is up to the protocol.  Since the
   default implementation returns None, it implicitly closes the connection.

   .. note::
      Some transports such as SSL don't support half-closed connections,
      in which case returning true from this method will not prevent closing
      the connection.

:meth:`data_received` can be called an arbitrary number of times during
a connection.  However, :meth:`eof_received` is called at most once
and, if called, :meth:`data_received` won't be called after it.

Datagram protocols
------------------

The following callbacks are called on :class:`DatagramProtocol` instances.

.. method:: DatagramProtocol.datagram_received(data, addr)

   Called when a datagram is received.  *data* is a bytes object containing
   the incoming data.  *addr* is the address of the peer sending the data;
   the exact format depends on the transport.

.. method:: DatagramProtocol.error_received(exc)

   Called when a previous send or receive operation raises an
   :class:`OSError`.  *exc* is the :class:`OSError` instance.

   This method is called in rare conditions, when the transport (e.g. UDP)
   detects that a datagram couldn't be delivered to its recipient.
   In many conditions though, undeliverable datagrams will be silently
   dropped.


Flow control callbacks
----------------------

These callbacks may be called on :class:`Protocol` and
:class:`SubprocessProtocol` instances:

.. method:: BaseProtocol.pause_writing()

   Called when the transport's buffer goes over the high-water mark.

.. method:: BaseProtocol.resume_writing()

   Called when the transport's buffer drains below the low-water mark.


:meth:`pause_writing` and :meth:`resume_writing` calls are paired --
:meth:`pause_writing` is called once when the buffer goes strictly over
the high-water mark (even if subsequent writes increases the buffer size
even more), and eventually :meth:`resume_writing` is called once when the
buffer size reaches the low-water mark.

.. note::
   If the buffer size equals the high-water mark,
   :meth:`pause_writing` is not called -- it must go strictly over.
   Conversely, :meth:`resume_writing` is called when the buffer size is
   equal or lower than the low-water mark.  These end conditions
   are important to ensure that things go as expected when either
   mark is zero.


Server
------

.. class:: AbstractServer

   Abstract server returned by create_service().

   .. method:: close()

      Stop serving.  This leaves existing connections open.

   .. method:: wait_closed()

      Coroutine to wait until service is closed.


Network functions
=================

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

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

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

   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
   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 returns a :ref:`coroutine <coroutine>`.

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

   Start a socket server, call back 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 <coroutine>`; if it is a coroutine, 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 returns a :ref:`coroutine <coroutine>`.


Protocol example: TCP echo client and server
============================================

Echo server
-----------

TCP echo server example::

    import asyncio

    class EchoServer(asyncio.Protocol):
        def timeout(self):
            print('connection timeout, closing.')
            self.transport.close()

        def connection_made(self, transport):
            print('connection made')
            self.transport = transport

            # close the client connection after 2 seconds
            asyncio.get_event_loop().call_later(2.0, self.timeout)

        def data_received(self, data):
            print('data received:', data.decode())
            self.transport.write(data)

        def connection_lost(self, exc):
            print('connection lost')


    loop = asyncio.get_event_loop()
    f = loop.create_server(EchoServer, '127.0.0.1', 8888)
    s = loop.run_until_complete(f)
    print('serving on', s.sockets[0].getsockname())
    loop.run_forever()


Echo client
-----------

TCP echo client example::

    import asyncio

    class EchoClient(asyncio.Protocol):
        message = 'This is the message. It will be echoed.'

        def connection_made(self, transport):
            self.transport = transport
            self.transport.write(self.message.encode())
            print('data sent:', self.message)

        def data_received(self, data):
            print('data received:', data.decode())

        def connection_lost(self, exc):
            print('connection lost')
            asyncio.get_event_loop().stop()

    loop = asyncio.get_event_loop()
    task = loop.create_connection(EchoClient, '127.0.0.1', 8888)
    loop.run_until_complete(task)
    loop.run_forever()
    loop.close()