xref: /third_party/python/Doc/library/asyncio-protocol.rst

.. currentmodule:: asyncio


.. _asyncio-transports-protocols:


========================
Transports and Protocols
========================

.. rubric:: Preface

Transports and Protocols are used by the **low-level** event loop
APIs such as :meth:`loop.create_connection`.  They use
callback-based programming style and enable high-performance
implementations of network or IPC protocols (e.g. HTTP).

Essentially, transports and protocols should only be used in
libraries and frameworks and never in high-level asyncio
applications.

This documentation page covers both `Transports`_ and `Protocols`_.

.. rubric:: Introduction

At the highest level, the transport is concerned with *how* bytes
are transmitted, while the protocol determines *which* bytes to
transmit (and to some extent when).

A different way of saying the same thing: a transport is an
abstraction for a socket (or similar I/O endpoint) while a protocol
is an abstraction for an application, from the transport's point
of view.

Yet another view is the transport and protocol interfaces
together define an abstract interface for using network I/O and
interprocess I/O.

There is always a 1:1 relationship between transport and protocol
objects: the protocol calls transport methods to send data,
while the transport calls protocol methods to pass it data that
has been received.

Most of connection oriented event loop methods
(such as :meth:`loop.create_connection`) usually accept a
*protocol_factory* argument used to create a *Protocol* object
for an accepted connection, represented by a *Transport* object.
Such methods usually return a tuple of ``(transport, protocol)``.

.. rubric:: Contents

This documentation page contains the following sections:

* The `Transports`_ section documents asyncio :class:`BaseTransport`,
  :class:`ReadTransport`, :class:`WriteTransport`, :class:`Transport`,
  :class:`DatagramTransport`, and :class:`SubprocessTransport`
  classes.

* The `Protocols`_ section documents asyncio :class:`BaseProtocol`,
  :class:`Protocol`, :class:`BufferedProtocol`,
  :class:`DatagramProtocol`, and :class:`SubprocessProtocol` classes.

* The `Examples`_ section showcases how to work with transports,
  protocols, and low-level event loop APIs.


.. _asyncio-transport:

Transports
==========

**Source code:** :source:`Lib/asyncio/transports.py`

----------------------------------------------------

Transports are classes provided by :mod:`asyncio` in order to abstract
various kinds of communication channels.

Transport objects are always instantiated by an
:ref:`asyncio event loop <asyncio-event-loop>`.

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

The transport classes are :ref:`not thread safe <asyncio-multithreading>`.


Transports Hierarchy
--------------------

.. class:: BaseTransport

   Base class for all transports.  Contains methods that all
   asyncio transports share.

.. class:: WriteTransport(BaseTransport)

   A base transport for write-only connections.

   Instances of the *WriteTransport* class are returned from
   the :meth:`loop.connect_write_pipe` event loop method and
   are also used by subprocess-related methods like
   :meth:`loop.subprocess_exec`.

.. class:: ReadTransport(BaseTransport)

   A base transport for read-only connections.

   Instances of the *ReadTransport* class are returned from
   the :meth:`loop.connect_read_pipe` event loop method and
   are also used by subprocess-related methods like
   :meth:`loop.subprocess_exec`.

.. class:: Transport(WriteTransport, ReadTransport)

   Interface representing a bidirectional transport, such as a
   TCP connection.

   The user does not instantiate a transport directly; they call a
   utility function, passing it a protocol factory and other
   information necessary to create the transport and protocol.

   Instances of the *Transport* class are returned from or used by
   event loop methods like :meth:`loop.create_connection`,
   :meth:`loop.create_unix_connection`,
   :meth:`loop.create_server`, :meth:`loop.sendfile`, etc.


.. class:: DatagramTransport(BaseTransport)

   A transport for datagram (UDP) connections.

   Instances of the *DatagramTransport* class are returned from
   the :meth:`loop.create_datagram_endpoint` event loop method.


.. class:: SubprocessTransport(BaseTransport)

   An abstraction to represent a connection between a parent and its
   child OS process.

   Instances of the *SubprocessTransport* class are returned from
   event loop methods :meth:`loop.subprocess_shell` and
   :meth:`loop.subprocess_exec`.


Base Transport
--------------

.. method:: BaseTransport.close()

   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:`protocol.connection_lost()
   <BaseProtocol.connection_lost>` method will be called with
   :const:`None` as its argument. The transport should not be
   used once it is closed.

.. method:: BaseTransport.is_closing()

   Return ``True`` if the transport is closing or is closed.

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

   Return information about the transport or underlying resources
   it uses.

   *name* is a string representing the piece of transport-specific
   information to get.

   *default* is the value to return if the information is not
   available, or if the transport does not support querying it
   with the given third-party event loop implementation or on the
   current platform.

   For example, the following code attempts to get the underlying
   socket object of the transport::

      sock = transport.get_extra_info('socket')
      if sock is not None:
          print(sock.getsockopt(...))

   Categories of information that can be queried on some transports:

   * 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

     - ``'ssl_object'``: :class:`ssl.SSLObject` or
       :class:`ssl.SSLSocket` instance

   * pipe:

     - ``'pipe'``: pipe object

   * subprocess:

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

.. method:: BaseTransport.set_protocol(protocol)

   Set a new protocol.

   Switching protocol should only be done when both
   protocols are documented to support the switch.

.. method:: BaseTransport.get_protocol()

   Return the current protocol.


Read-only Transports
--------------------

.. method:: ReadTransport.is_reading()

   Return ``True`` if the transport is receiving new data.

   .. versionadded:: 3.7

.. method:: ReadTransport.pause_reading()

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

   .. versionchanged:: 3.7
      The method is idempotent, i.e. it can be called when the
      transport is already paused or closed.

.. method:: ReadTransport.resume_reading()

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

   .. versionchanged:: 3.7
      The method is idempotent, i.e. it can be called when the
      transport is already reading.


Write-only Transports
---------------------

.. method:: WriteTransport.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:`protocol.connection_lost()
   <BaseProtocol.connection_lost>` method will eventually be
   called with :const:`None` as its argument.

.. method:: WriteTransport.can_write_eof()

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

.. method:: WriteTransport.get_write_buffer_size()

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

.. method:: WriteTransport.get_write_buffer_limits()

   Get the *high* and *low* watermarks for write flow control. Return a
   tuple ``(low, high)`` where *low* and *high* are positive number of
   bytes.

   Use :meth:`set_write_buffer_limits` to set the limits.

   .. versionadded:: 3.4.2

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

   Set the *high* and *low* watermarks for write flow control.

   These two values (measured in number of
   bytes) control when the protocol's
   :meth:`protocol.pause_writing() <BaseProtocol.pause_writing>`
   and :meth:`protocol.resume_writing() <BaseProtocol.resume_writing>`
   methods are called. If specified, the low watermark must be less
   than or equal to the high watermark.  Neither *high* nor *low*
   can be negative.

   :meth:`~BaseProtocol.pause_writing` is called when the buffer size
   becomes greater than or equal to the *high* value. If writing has
   been paused, :meth:`~BaseProtocol.resume_writing` is called when
   the buffer size becomes less than or equal to the *low* value.

   The defaults are implementation-specific.  If only the
   high watermark is given, the low watermark defaults to an
   implementation-specific value less than or equal to the
   high watermark.  Setting *high* to zero forces *low* to zero as
   well, and causes :meth:`~BaseProtocol.pause_writing` to be called
   whenever the buffer becomes non-empty.  Setting *low* to zero causes
   :meth:`~BaseProtocol.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.

   Use :meth:`~WriteTransport.get_write_buffer_limits`
   to get the limits.

.. method:: WriteTransport.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:: WriteTransport.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:: WriteTransport.write_eof()

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

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


Datagram Transports
-------------------

.. 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:`protocol.connection_lost() <BaseProtocol.connection_lost>`
   method will eventually be called with :const:`None` as its argument.


.. _asyncio-subprocess-transports:

Subprocess Transports
---------------------

.. method:: SubprocessTransport.get_pid()

   Return the subprocess process id as an integer.

.. method:: SubprocessTransport.get_pipe_transport(fd)

   Return the transport for the communication pipe corresponding to the
   integer file descriptor *fd*:

   * ``0``: readable streaming transport of the standard input (*stdin*),
     or :const:`None` if the subprocess was not created with ``stdin=PIPE``
   * ``1``: writable streaming transport of the standard output (*stdout*),
     or :const:`None` if the subprocess was not created with ``stdout=PIPE``
   * ``2``: writable streaming transport of the standard error (*stderr*),
     or :const:`None` if the subprocess was not created with ``stderr=PIPE``
   * other *fd*: :const:`None`

.. method:: SubprocessTransport.get_returncode()

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

.. method:: SubprocessTransport.kill()

   Kill the subprocess.

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

   See also :meth:`subprocess.Popen.kill`.

.. method:: SubprocessTransport.send_signal(signal)

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

.. method:: SubprocessTransport.terminate()

   Stop the subprocess.

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

   See also :meth:`subprocess.Popen.terminate`.

.. method:: SubprocessTransport.close()

   Kill the subprocess by calling the :meth:`kill` method.

   If the subprocess hasn't returned yet, and close transports of
   *stdin*, *stdout*, and *stderr* pipes.


.. _asyncio-protocol:

Protocols
=========

**Source code:** :source:`Lib/asyncio/protocols.py`

---------------------------------------------------

asyncio provides a set of abstract base classes that should be used
to implement network protocols.  Those classes are meant to be used
together with :ref:`transports <asyncio-transport>`.

Subclasses of abstract base protocol classes may implement some or
all methods.  All these methods are callbacks: they are called by
transports on certain events, for example when some data is received.
A base protocol method should be called by the corresponding transport.


Base Protocols
--------------

.. class:: BaseProtocol

   Base protocol with methods that all protocols share.

.. class:: Protocol(BaseProtocol)

   The base class for implementing streaming protocols
   (TCP, Unix sockets, etc).

.. class:: BufferedProtocol(BaseProtocol)

   A base class for implementing streaming protocols with manual
   control of the receive buffer.

.. class:: DatagramProtocol(BaseProtocol)

   The base class for implementing datagram (UDP) protocols.

.. class:: SubprocessProtocol(BaseProtocol)

   The base class for implementing protocols communicating with child
   processes (unidirectional pipes).


Base Protocol
-------------

All asyncio protocols can implement Base Protocol callbacks.

.. rubric:: Connection Callbacks

Connection callbacks are called on all protocols, exactly once per
a successful connection.  All other protocol callbacks can only be
called between those two methods.

.. method:: BaseProtocol.connection_made(transport)

   Called when a connection is made.

   The *transport* argument is the transport representing the
   connection.  The protocol is responsible for storing the reference
   to its transport.

.. 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.


.. rubric:: Flow Control Callbacks

Flow control callbacks can be called by transports to pause or
resume writing performed by the protocol.

See the documentation of the :meth:`~WriteTransport.set_write_buffer_limits`
method for more details.

.. method:: BaseProtocol.pause_writing()

   Called when the transport's buffer goes over the high watermark.

.. method:: BaseProtocol.resume_writing()

   Called when the transport's buffer drains below the low watermark.

If the buffer size equals the high watermark,
:meth:`~BaseProtocol.pause_writing` is not called: the buffer size must
go strictly over.

Conversely, :meth:`~BaseProtocol.resume_writing` is called when the
buffer size is equal or lower than the low watermark.  These end
conditions are important to ensure that things go as expected when
either mark is zero.


Streaming Protocols
-------------------

Event methods, such as :meth:`loop.create_server`,
:meth:`loop.create_unix_server`, :meth:`loop.create_connection`,
:meth:`loop.create_unix_connection`, :meth:`loop.connect_accepted_socket`,
:meth:`loop.connect_read_pipe`, and :meth:`loop.connect_write_pipe`
accept factories that return streaming protocols.

.. method:: Protocol.data_received(data)

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

   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. However,
   data is always received in the correct order.

   The method can be called an arbitrary number of times while
   a connection is open.

   However, :meth:`protocol.eof_received() <Protocol.eof_received>`
   is called at most once.  Once ``eof_received()`` is called,
   ``data_received()`` is not called anymore.

.. method:: Protocol.eof_received()

   Called when the other end signals it won't send any more data
   (for example by calling :meth:`transport.write_eof()
   <WriteTransport.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, the protocol used determines whether to close the transport.
   Since the default implementation returns ``None``, it implicitly closes the
   connection.

   Some transports, including SSL, don't support half-closed connections,
   in which case returning true from this method will result in the connection
   being closed.


State machine:

.. code-block:: none

    start -> connection_made
        [-> data_received]*
        [-> eof_received]?
    -> connection_lost -> end


Buffered Streaming Protocols
----------------------------

.. versionadded:: 3.7

Buffered Protocols can be used with any event loop method
that supports `Streaming Protocols`_.

``BufferedProtocol`` implementations allow explicit manual allocation
and control of the receive buffer.  Event loops can then use the buffer
provided by the protocol to avoid unnecessary data copies.  This
can result in noticeable performance improvement for protocols that
receive big amounts of data.  Sophisticated protocol implementations
can significantly reduce the number of buffer allocations.

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

.. method:: BufferedProtocol.get_buffer(sizehint)

   Called to allocate a new receive buffer.

   *sizehint* is the recommended minimum size for the returned
   buffer.  It is acceptable to return smaller or larger buffers
   than what *sizehint* suggests.  When set to -1, the buffer size
   can be arbitrary. It is an error to return a buffer with a zero size.

   ``get_buffer()`` must return an object implementing the
   :ref:`buffer protocol <bufferobjects>`.

.. method:: BufferedProtocol.buffer_updated(nbytes)

   Called when the buffer was updated with the received data.

   *nbytes* is the total number of bytes that were written to the buffer.

.. method:: BufferedProtocol.eof_received()

   See the documentation of the :meth:`protocol.eof_received()
   <Protocol.eof_received>` method.


:meth:`~BufferedProtocol.get_buffer` can be called an arbitrary number
of times during a connection.  However, :meth:`protocol.eof_received()
<Protocol.eof_received>` is called at most once
and, if called, :meth:`~BufferedProtocol.get_buffer` and
:meth:`~BufferedProtocol.buffer_updated` won't be called after it.

State machine:

.. code-block:: none

    start -> connection_made
        [-> get_buffer
            [-> buffer_updated]?
        ]*
        [-> eof_received]?
    -> connection_lost -> end


Datagram Protocols
------------------

Datagram Protocol instances should be constructed by protocol
factories passed to the :meth:`loop.create_datagram_endpoint` method.

.. 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 could not be delivered to its recipient.
   In many conditions though, undeliverable datagrams will be silently
   dropped.

.. note::

   On BSD systems (macOS, FreeBSD, etc.) flow control is not supported
   for datagram protocols, because there is no reliable way to detect send
   failures caused by writing too many packets.

   The socket always appears 'ready' and excess packets are dropped. An
   :class:`OSError` with ``errno`` set to :const:`errno.ENOBUFS` may
   or may not be raised; if it is raised, it will be reported to
   :meth:`DatagramProtocol.error_received` but otherwise ignored.


.. _asyncio-subprocess-protocols:

Subprocess Protocols
--------------------

Subprocess Protocol instances should be constructed by protocol
factories passed to the :meth:`loop.subprocess_exec` and
:meth:`loop.subprocess_shell` methods.

.. 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 received 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.


Examples
========

.. _asyncio_example_tcp_echo_server_protocol:

TCP Echo Server
---------------

Create a TCP echo server using the :meth:`loop.create_server` method, send back
received data, and close the connection::

    import asyncio


    class EchoServerProtocol(asyncio.Protocol):
        def connection_made(self, transport):
            peername = transport.get_extra_info('peername')
            print('Connection from {}'.format(peername))
            self.transport = transport

        def data_received(self, data):
            message = data.decode()
            print('Data received: {!r}'.format(message))

            print('Send: {!r}'.format(message))
            self.transport.write(data)

            print('Close the client socket')
            self.transport.close()


    async def main():
        # Get a reference to the event loop as we plan to use
        # low-level APIs.
        loop = asyncio.get_running_loop()

        server = await loop.create_server(
            lambda: EchoServerProtocol(),
            '127.0.0.1', 8888)

        async with server:
            await server.serve_forever()


    asyncio.run(main())


.. seealso::

   The :ref:`TCP echo server using streams <asyncio-tcp-echo-server-streams>`
   example uses the high-level :func:`asyncio.start_server` function.

.. _asyncio_example_tcp_echo_client_protocol:

TCP Echo Client
---------------

A TCP echo client using the :meth:`loop.create_connection` method, sends
data, and waits until the connection is closed::

    import asyncio


    class EchoClientProtocol(asyncio.Protocol):
        def __init__(self, message, on_con_lost):
            self.message = message
            self.on_con_lost = on_con_lost

        def connection_made(self, transport):
            transport.write(self.message.encode())
            print('Data sent: {!r}'.format(self.message))

        def data_received(self, data):
            print('Data received: {!r}'.format(data.decode()))

        def connection_lost(self, exc):
            print('The server closed the connection')
            self.on_con_lost.set_result(True)


    async def main():
        # Get a reference to the event loop as we plan to use
        # low-level APIs.
        loop = asyncio.get_running_loop()

        on_con_lost = loop.create_future()
        message = 'Hello World!'

        transport, protocol = await loop.create_connection(
            lambda: EchoClientProtocol(message, on_con_lost),
            '127.0.0.1', 8888)

        # Wait until the protocol signals that the connection
        # is lost and close the transport.
        try:
            await on_con_lost
        finally:
            transport.close()


    asyncio.run(main())


.. seealso::

   The :ref:`TCP echo client using streams <asyncio-tcp-echo-client-streams>`
   example uses the high-level :func:`asyncio.open_connection` function.


.. _asyncio-udp-echo-server-protocol:

UDP Echo Server
---------------

A UDP echo server, using the :meth:`loop.create_datagram_endpoint`
method, sends back received data::

    import asyncio


    class EchoServerProtocol:
        def connection_made(self, transport):
            self.transport = transport

        def datagram_received(self, data, addr):
            message = data.decode()
            print('Received %r from %s' % (message, addr))
            print('Send %r to %s' % (message, addr))
            self.transport.sendto(data, addr)


    async def main():
        print("Starting UDP server")

        # Get a reference to the event loop as we plan to use
        # low-level APIs.
        loop = asyncio.get_running_loop()

        # One protocol instance will be created to serve all
        # client requests.
        transport, protocol = await loop.create_datagram_endpoint(
            lambda: EchoServerProtocol(),
            local_addr=('127.0.0.1', 9999))

        try:
            await asyncio.sleep(3600)  # Serve for 1 hour.
        finally:
            transport.close()


    asyncio.run(main())


.. _asyncio-udp-echo-client-protocol:

UDP Echo Client
---------------

A UDP echo client, using the :meth:`loop.create_datagram_endpoint`
method, sends data and closes the transport when it receives the answer::

    import asyncio


    class EchoClientProtocol:
        def __init__(self, message, on_con_lost):
            self.message = message
            self.on_con_lost = on_con_lost
            self.transport = None

        def connection_made(self, transport):
            self.transport = transport
            print('Send:', self.message)
            self.transport.sendto(self.message.encode())

        def datagram_received(self, data, addr):
            print("Received:", data.decode())

            print("Close the socket")
            self.transport.close()

        def error_received(self, exc):
            print('Error received:', exc)

        def connection_lost(self, exc):
            print("Connection closed")
            self.on_con_lost.set_result(True)


    async def main():
        # Get a reference to the event loop as we plan to use
        # low-level APIs.
        loop = asyncio.get_running_loop()

        on_con_lost = loop.create_future()
        message = "Hello World!"

        transport, protocol = await loop.create_datagram_endpoint(
            lambda: EchoClientProtocol(message, on_con_lost),
            remote_addr=('127.0.0.1', 9999))

        try:
            await on_con_lost
        finally:
            transport.close()


    asyncio.run(main())


.. _asyncio_example_create_connection:

Connecting Existing Sockets
---------------------------

Wait until a socket receives data using the
:meth:`loop.create_connection` method with a protocol::

    import asyncio
    import socket


    class MyProtocol(asyncio.Protocol):

        def __init__(self, on_con_lost):
            self.transport = None
            self.on_con_lost = on_con_lost

        def connection_made(self, transport):
            self.transport = transport

        def data_received(self, data):
            print("Received:", data.decode())

            # We are done: close the transport;
            # connection_lost() will be called automatically.
            self.transport.close()

        def connection_lost(self, exc):
            # The socket has been closed
            self.on_con_lost.set_result(True)


    async def main():
        # Get a reference to the event loop as we plan to use
        # low-level APIs.
        loop = asyncio.get_running_loop()
        on_con_lost = loop.create_future()

        # Create a pair of connected sockets
        rsock, wsock = socket.socketpair()

        # Register the socket to wait for data.
        transport, protocol = await loop.create_connection(
            lambda: MyProtocol(on_con_lost), sock=rsock)

        # Simulate the reception of data from the network.
        loop.call_soon(wsock.send, 'abc'.encode())

        try:
            await protocol.on_con_lost
        finally:
            transport.close()
            wsock.close()

    asyncio.run(main())

.. seealso::

   The :ref:`watch a file descriptor for read events
   <asyncio_example_watch_fd>` example uses the low-level
   :meth:`loop.add_reader` method to register an FD.

   The :ref:`register an open socket to wait for data using streams
   <asyncio_example_create_connection-streams>` example uses high-level streams
   created by the :func:`open_connection` function in a coroutine.

.. _asyncio_example_subprocess_proto:

loop.subprocess_exec() and SubprocessProtocol
---------------------------------------------

An example of a subprocess protocol used to get the output of a
subprocess and to wait for the subprocess exit.

The subprocess is created by the :meth:`loop.subprocess_exec` method::

    import asyncio
    import sys

    class DateProtocol(asyncio.SubprocessProtocol):
        def __init__(self, exit_future):
            self.exit_future = exit_future
            self.output = bytearray()

        def pipe_data_received(self, fd, data):
            self.output.extend(data)

        def process_exited(self):
            self.exit_future.set_result(True)

    async def get_date():
        # Get a reference to the event loop as we plan to use
        # low-level APIs.
        loop = asyncio.get_running_loop()

        code = 'import datetime; print(datetime.datetime.now())'
        exit_future = asyncio.Future(loop=loop)

        # Create the subprocess controlled by DateProtocol;
        # redirect the standard output into a pipe.
        transport, protocol = await loop.subprocess_exec(
            lambda: DateProtocol(exit_future),
            sys.executable, '-c', code,
            stdin=None, stderr=None)

        # Wait for the subprocess exit using the process_exited()
        # method of the protocol.
        await exit_future

        # Close the stdout pipe.
        transport.close()

        # Read the output which was collected by the
        # pipe_data_received() method of the protocol.
        data = bytes(protocol.output)
        return data.decode('ascii').rstrip()

    date = asyncio.run(get_date())
    print(f"Current date: {date}")

See also the :ref:`same example <asyncio_example_create_subprocess_exec>`
written using high-level APIs.