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Doc/library/asyncio-stream.rst
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Kumar Aditya
gh-75279: Document that asyncio StreamWriter.drain() may not yield to the event loop (#155132)
03 авг 2026, 21:07
03 авг 2026, 21:07ab8c737
.. currentmodule:: asyncio

.. _asyncio-streams:

=======
Streams
=======

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

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

Streams are high-level async/await-ready primitives to work with
network connections.  Streams allow sending and receiving data without
using callbacks or low-level protocols and transports.

.. _asyncio_example_stream:

Here is an example of a TCP echo client written using asyncio
streams::

    import asyncio

    async def tcp_echo_client(message):
        reader, writer = await asyncio.open_connection(
            '127.0.0.1', 8888)

        print(f'Send: {message!r}')
        writer.write(message.encode())
        await writer.drain()

        data = await reader.read(100)
        print(f'Received: {data.decode()!r}')

        print('Close the connection')
        writer.close()
        await writer.wait_closed()

    asyncio.run(tcp_echo_client('Hello World!'))


See also the `Examples`_ section below.


.. rubric:: Stream Functions

The following top-level asyncio functions can be used to create
and work with streams:


.. function:: open_connection(host=None, port=None, *, \
                 limit=None, ssl=None, family=0, proto=0, \
                 flags=0, sock=None, local_addr=None, \
                 server_hostname=None, ssl_handshake_timeout=None, \
                 ssl_shutdown_timeout=None, \
                 happy_eyeballs_delay=None, interleave=None)
   :async:

   Establish a network connection and return a pair of
   ``(reader, writer)`` objects.

   The returned *reader* and *writer* objects are instances of
   :class:`StreamReader` and :class:`StreamWriter` classes.

   *limit* determines the buffer size limit used by the
   returned :class:`StreamReader` instance.  By default the *limit*
   is set to 64 KiB.

   The rest of the arguments are passed directly to
   :meth:`loop.create_connection`.

   .. note::

      The *sock* argument transfers ownership of the socket to the
      :class:`StreamWriter` created. To close the socket, call its
      :meth:`~asyncio.StreamWriter.close` method.

   .. versionchanged:: 3.7
      Added the *ssl_handshake_timeout* parameter.

   .. versionchanged:: 3.8
      Added the *happy_eyeballs_delay* and *interleave* parameters.

   .. versionchanged:: 3.10
      Removed the *loop* parameter.

   .. versionchanged:: 3.11
      Added the *ssl_shutdown_timeout* parameter.


.. function:: start_server(client_connected_cb, host=None, \
                 port=None, *, limit=None, \
                 family=socket.AF_UNSPEC, \
                 flags=socket.AI_PASSIVE, sock=None, \
                 backlog=100, ssl=None, reuse_address=None, \
                 reuse_port=None, keep_alive=None, \
                 ssl_handshake_timeout=None, \
                 ssl_shutdown_timeout=None, start_serving=True)
   :async:

   Start a socket server.

   The *client_connected_cb* callback is called whenever a new client
   connection is established.  It receives a ``(reader, writer)`` pair
   as two arguments, instances of the :class:`StreamReader` and
   :class:`StreamWriter` classes.

   *client_connected_cb* can be a plain callable or a
   :ref:`coroutine function <coroutine>`; if it is a coroutine function,
   it will be automatically scheduled as a :class:`Task`.

   *limit* determines the buffer size limit used by the
   returned :class:`StreamReader` instance.  By default the *limit*
   is set to 64 KiB.

   The rest of the arguments are passed directly to
   :meth:`loop.create_server`.

   .. note::

      The *sock* argument transfers ownership of the socket to the
      server created. To close the socket, call the server's
      :meth:`~asyncio.Server.close` method.

   .. versionchanged:: 3.7
      Added the *ssl_handshake_timeout* and *start_serving* parameters.

   .. versionchanged:: 3.10
      Removed the *loop* parameter.

   .. versionchanged:: 3.11
      Added the *ssl_shutdown_timeout* parameter.

   .. versionchanged:: 3.13
      Added the *keep_alive* parameter.


.. rubric:: Unix Sockets

.. function:: open_unix_connection(path=None, *, limit=None, \
               ssl=None, sock=None, server_hostname=None, \
               ssl_handshake_timeout=None, ssl_shutdown_timeout=None)
   :async:

   Establish a Unix socket connection and return a pair of
   ``(reader, writer)``.

   Similar to :func:`open_connection` but operates on Unix sockets.

   See also the documentation of :meth:`loop.create_unix_connection`.

   .. note::

      The *sock* argument transfers ownership of the socket to the
      :class:`StreamWriter` created. To close the socket, call its
      :meth:`~asyncio.StreamWriter.close` method.

   .. availability:: Unix.

   .. versionchanged:: 3.7
      Added the *ssl_handshake_timeout* parameter.
      The *path* parameter can now be a :term:`path-like object`

   .. versionchanged:: 3.10
      Removed the *loop* parameter.

   .. versionchanged:: 3.11
      Added the *ssl_shutdown_timeout* parameter.


.. function:: start_unix_server(client_connected_cb, path=None, \
                 *, limit=None, sock=None, backlog=100, ssl=None, \
                 ssl_handshake_timeout=None, \
                 ssl_shutdown_timeout=None, start_serving=True, \
                 cleanup_socket=True, mode=None)
   :async:

   Start a Unix socket server.

   Similar to :func:`start_server` but works with Unix sockets.

   If *cleanup_socket* is true then the Unix socket will automatically
   be removed from the filesystem when the server is closed, unless the
   socket has been replaced after the server has been created.

   If *mode* is not ``None``, the permissions of the Unix socket file
   are set to *mode* before the server starts accepting connections.

   See also the documentation of :meth:`loop.create_unix_server`.

   .. note::

      The *sock* argument transfers ownership of the socket to the
      server created. To close the socket, call the server's
      :meth:`~asyncio.Server.close` method.

   .. availability:: Unix.

   .. versionchanged:: 3.7
      Added the *ssl_handshake_timeout* and *start_serving* parameters.
      The *path* parameter can now be a :term:`path-like object`.

   .. versionchanged:: 3.10
      Removed the *loop* parameter.

   .. versionchanged:: 3.11
      Added the *ssl_shutdown_timeout* parameter.

   .. versionchanged:: 3.13
      Added the *cleanup_socket* parameter.

   .. versionchanged:: 3.16
      Added the *mode* parameter.


StreamReader
============

.. class:: StreamReader

   Represents a reader object that provides APIs to read data
   from the IO stream. As an :term:`asynchronous iterable`, the
   object supports the :keyword:`async for` statement.

   It is not recommended to instantiate *StreamReader* objects
   directly; use :func:`open_connection` and :func:`start_server`
   instead.

   .. method:: feed_eof()

      Acknowledge the EOF.

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

      Read up to *n* bytes from the stream.

      If *n* is not provided or set to ``-1``,
      read until EOF, then return all read :class:`bytes`.
      If EOF was received and the internal buffer is empty,
      return an empty ``bytes`` object.

      If *n* is ``0``, return an empty ``bytes`` object immediately.

      If *n* is positive, return at most *n* available ``bytes``
      as soon as at least 1 byte is available in the internal buffer.
      If EOF is received before any byte is read, return an empty
      ``bytes`` object.

   .. method:: readline()
      :async:

      Read one line, where "line" is a sequence of bytes
      ending with ``\n``.

      If EOF is received and ``\n`` was not found, the method
      returns partially read data.

      If EOF is received and the internal buffer is empty,
      return an empty ``bytes`` object.

   .. method:: readexactly(n)
      :async:

      Read exactly *n* bytes.

      Raise an :exc:`IncompleteReadError` if EOF is reached before *n*
      can be read.  Use the :attr:`IncompleteReadError.partial`
      attribute to get the partially read data.

   .. method:: readuntil(separator=b'\n')
      :async:

      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.

      If the amount of data read exceeds the configured stream limit, a
      :exc:`LimitOverrunError` exception is raised, and the data
      is left in the internal buffer and can be read again.

      If EOF is reached before the complete separator is found,
      an :exc:`IncompleteReadError` exception is raised, and the internal
      buffer is reset.  The :attr:`IncompleteReadError.partial` attribute
      may contain a portion of the separator.

      The *separator* may also be a tuple of separators. In this
      case the return value will be the shortest possible that has any
      separator as the suffix. For the purposes of :exc:`LimitOverrunError`,
      the shortest possible separator is considered to be the one that
      matched.

      .. versionadded:: 3.5.2

      .. versionchanged:: 3.13

         The *separator* parameter may now be a :class:`tuple` of
         separators.

   .. method:: at_eof()

      Return ``True`` if the buffer is empty and :meth:`feed_eof`
      was called.


StreamWriter
============

.. class:: StreamWriter

   Represents a writer object that provides APIs to write data
   to the IO stream.

   It is not recommended to instantiate *StreamWriter* objects
   directly; use :func:`open_connection` and :func:`start_server`
   instead.

   .. method:: write(data)

      The method attempts to write the *data* to the underlying socket immediately.
      If that fails, the data is queued in an internal write buffer until it can be
      sent.

      The *data* buffer should be a bytes, bytearray, or C-contiguous one-dimensional
      memoryview object.

      The method should be used along with the ``drain()`` method::

         stream.write(data)
         await stream.drain()


   .. method:: writelines(data)

      The method writes a list (or any iterable) of bytes to the underlying socket
      immediately.
      If that fails, the data is queued in an internal write buffer until it can be
      sent.

      The method should be used along with the ``drain()`` method::

         stream.writelines(lines)
         await stream.drain()

   .. method:: close()

      The method closes the stream and the underlying socket.

      The method should be used, though not mandatory,
      along with the ``wait_closed()`` method::

         stream.close()
         await stream.wait_closed()

   .. method:: can_write_eof()

      Return ``True`` if the underlying transport supports
      the :meth:`write_eof` method, ``False`` otherwise.

   .. method:: write_eof()

      Close the write end of the stream after the buffered write
      data is flushed.

   .. attribute:: transport

      Return the underlying asyncio transport.

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

      Access optional transport information; see
      :meth:`BaseTransport.get_extra_info` for details.

   .. method:: drain()
      :async:

      Wait until it is appropriate to resume writing to the stream.
      Example::

          writer.write(data)
          await writer.drain()

      This is a flow control method that interacts with the underlying
      IO write buffer.  When the size of the buffer reaches
      the high watermark, *drain()* blocks until the size of the
      buffer is drained down to the low watermark and writing can
      be resumed.  When there is nothing to wait for, the :meth:`drain`
      returns immediately.

      .. note::

         When the write buffer is below the high watermark,
         :meth:`drain` returns immediately without yielding to
         the event loop.  As a result, code which repeatedly calls
         ``write()`` followed by ``await drain()`` may prevent other
         tasks from running.  To prevent blocking behavior, yield
         to the event loop explicitly with ``await asyncio.sleep(0)``
         (see :func:`asyncio.sleep`).

   .. method:: start_tls(sslcontext, *, server_hostname=None, \
                         ssl_handshake_timeout=None, ssl_shutdown_timeout=None)
      :async:

      Upgrade an existing stream-based connection to TLS.

      Parameters:

      * *sslcontext*: a configured instance of :class:`~ssl.SSLContext`.

      * *server_hostname*: sets or overrides the host name that the target
        server's certificate will be matched against.

      * *ssl_handshake_timeout* is the time in seconds to wait for the TLS
        handshake to complete before aborting the connection.  ``60.0`` seconds
        if ``None`` (default).

      * *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown
        to complete before aborting the connection. ``30.0`` seconds if ``None``
        (default).

      .. versionadded:: 3.11

      .. versionchanged:: 3.12
         Added the *ssl_shutdown_timeout* parameter.


   .. method:: is_closing()

      Return ``True`` if the stream is closed or in the process of
      being closed.

      .. versionadded:: 3.7

   .. method:: wait_closed()
      :async:

      Wait until the stream is closed.

      Should be called after :meth:`close` to wait until the underlying
      connection is closed, ensuring that all data has been flushed
      before e.g. exiting the program.

      .. versionadded:: 3.7


Examples
========

.. _asyncio-tcp-echo-client-streams:

TCP echo client using streams
-----------------------------

TCP echo client using the :func:`asyncio.open_connection` function::

    import asyncio

    async def tcp_echo_client(message):
        reader, writer = await asyncio.open_connection(
            '127.0.0.1', 8888)

        print(f'Send: {message!r}')
        writer.write(message.encode())
        await writer.drain()

        data = await reader.read(100)
        print(f'Received: {data.decode()!r}')

        print('Close the connection')
        writer.close()
        await writer.wait_closed()

    asyncio.run(tcp_echo_client('Hello World!'))


.. seealso::

   The :ref:`TCP echo client protocol <asyncio_example_tcp_echo_client_protocol>`
   example uses the low-level :meth:`loop.create_connection` method.


.. _asyncio-tcp-echo-server-streams:

TCP echo server using streams
-----------------------------

TCP echo server using the :func:`asyncio.start_server` function::

    import asyncio

    async def handle_echo(reader, writer):
        data = await reader.read(100)
        message = data.decode()
        addr = writer.get_extra_info('peername')

        print(f"Received {message!r} from {addr!r}")

        print(f"Send: {message!r}")
        writer.write(data)
        await writer.drain()

        print("Close the connection")
        writer.close()
        await writer.wait_closed()

    async def main():
        server = await asyncio.start_server(
            handle_echo, '127.0.0.1', 8888)

        addrs = ', '.join(str(sock.getsockname()) for sock in server.sockets)
        print(f'Serving on {addrs}')

        async with server:
            await server.serve_forever()

    asyncio.run(main())


.. seealso::

   The :ref:`TCP echo server protocol <asyncio_example_tcp_echo_server_protocol>`
   example uses the :meth:`loop.create_server` method.


Get HTTP headers
----------------

Simple example querying HTTP headers of the URL passed on the command line::

    import asyncio
    import urllib.parse
    import sys

    async def print_http_headers(url):
        url = urllib.parse.urlsplit(url)
        if url.scheme == 'https':
            reader, writer = await asyncio.open_connection(
                url.hostname, 443, ssl=True)
        else:
            reader, writer = await asyncio.open_connection(
                url.hostname, 80)

        query = (
            f"HEAD {url.path or '/'} HTTP/1.0\r\n"
            f"Host: {url.hostname}\r\n"
            f"\r\n"
        )

        writer.write(query.encode('latin-1'))
        while True:
            line = await reader.readline()
            if not line:
                break

            line = line.decode('latin1').rstrip()
            if line:
                print(f'HTTP header> {line}')

        # Ignore the body, close the socket
        writer.close()
        await writer.wait_closed()

    url = sys.argv[1]
    asyncio.run(print_http_headers(url))


Usage::

    python example.py http://example.com/path/page.html

or with HTTPS::

    python example.py https://example.com/path/page.html


.. _asyncio_example_create_connection-streams:

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

Coroutine waiting until a socket receives data using the
:func:`open_connection` function::

    import asyncio
    import socket

    async def wait_for_data():
        # Get a reference to the current event loop because
        # we want to access low-level APIs.
        loop = asyncio.get_running_loop()

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

        # Register the open socket to wait for data.
        reader, writer = await asyncio.open_connection(sock=rsock)

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

        # Wait for data
        data = await reader.read(100)

        # Got data, we are done: close the socket
        print("Received:", data.decode())
        writer.close()
        await writer.wait_closed()

        # Close the second socket
        wsock.close()

    asyncio.run(wait_for_data())

.. seealso::

   The :ref:`register an open socket to wait for data using a protocol
   <asyncio_example_create_connection>` example uses a low-level protocol and
   the :meth:`loop.create_connection` method.

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