.. currentmodule:: asyncio

.. _asyncio-streams:

======= Streams

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())

    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:

.. coroutinefunction:: open_connection(host=None, port=None, *,
loop=None, limit=None, ssl=None, family=0,
proto=0, flags=0, sock=None, local_addr=None,
server_hostname=None, ssl_handshake_timeout=None)

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.

The loop argument is optional and can always be determined automatically when this function is awaited from a coroutine.

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.

.. versionadded:: 3.7

  The *ssl_handshake_timeout* parameter.

.. coroutinefunction:: start_server(client_connected_cb, host=None,
port=None, *, loop=None, limit=None,
family=socket.AF_UNSPEC,
flags=socket.AI_PASSIVE, sock=None,
backlog=100, ssl=None, reuse_address=None,
reuse_port=None, ssl_handshake_timeout=None,
start_serving=True)

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.

The loop argument is optional and can always be determined automatically when this method is awaited from a coroutine.

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.

.. versionadded:: 3.7

  The *ssl_handshake_timeout* and *start_serving* parameters.

.. rubric:: Unix Sockets

.. coroutinefunction:: open_unix_connection(path=None, *, loop=None,
limit=None, ssl=None, sock=None,
server_hostname=None, ssl_handshake_timeout=None)

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.

.. availability:: Unix.

.. versionadded:: 3.7

  The *ssl_handshake_timeout* parameter.

.. versionchanged:: 3.7

  The *path* parameter can now be a :term:`path-like object`

.. coroutinefunction:: start_unix_server(client_connected_cb, path=None,
*, loop=None, limit=None, sock=None,
backlog=100, ssl=None, ssl_handshake_timeout=None,
start_serving=True)

Start a Unix socket server.

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

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

.. availability:: Unix.

.. versionadded:: 3.7

  The *ssl_handshake_timeout* and *start_serving* parameters.

.. versionchanged:: 3.7

  The *path* parameter can now be a :term:`path-like object`.

StreamReader

.. class:: StreamReader

Represents a reader object that provides APIs to read data from the IO stream.

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

.. coroutinemethod:: 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 EOF was received and the internal buffer is empty,
  return an empty ``bytes`` object.

.. coroutinemethod:: 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
  returns partially read data.

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

.. coroutinemethod:: readexactly(n)

  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.

.. coroutinemethod:: 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.

  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.

  .. versionadded:: 3.5.2

.. 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:: 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:: write(data)

  Write *data* to the stream.

  This method is not subject to flow control.  Calls to ``write()`` should
  be followed by :meth:`drain`.

.. method:: writelines(data)

  Write a list (or any iterable) of bytes to the stream.

  This method is not subject to flow control. Calls to ``writelines()``
  should be followed by :meth:`drain`.

.. coroutinemethod:: drain()

  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.

.. method:: close()

  Close the stream.

.. method:: is_closing()

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

  .. versionadded:: 3.7

.. coroutinemethod:: wait_closed()

  Wait until the stream is closed.

  Should be called after :meth:`close` to wait until the underlying
  connection is closed.

  .. 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())

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

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

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()

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

    addr = server.sockets[0].getsockname()
    print(f'Serving on {addr}')

    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()

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()

    # 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.