Streams
*******

**Source code:** 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.

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.

-[ Stream Functions ]-

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

coroutine asyncio.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
   "StreamReader" and "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
   "StreamReader" instance.  By default the *limit* is set to 64 KiB.

   The rest of the arguments are passed directly to
   "loop.create_connection()".

   New in version 3.7: The *ssl_handshake_timeout* parameter.

coroutine asyncio.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 "StreamReader" and
   "StreamWriter" classes.

   *client_connected_cb* can be a plain callable or a coroutine
   function; if it is a coroutine function, it will be automatically
   scheduled as a "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
   "StreamReader" instance.  By default the *limit* is set to 64 KiB.

   The rest of the arguments are passed directly to
   "loop.create_server()".

   New in version 3.7: The *ssl_handshake_timeout* and *start_serving*
   parameters.

-[ Unix Sockets ]-

coroutine asyncio.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 "open_connection()" but operates on Unix sockets.

   See also the documentation of "loop.create_unix_connection()".

   Availability: Unix.

   New in version 3.7: The *ssl_handshake_timeout* parameter.

   Changed in version 3.7: The *path* parameter can now be a *path-
   like object*

coroutine asyncio.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 "start_server()" but works with Unix sockets.

   See also the documentation of "loop.create_unix_server()".

   Availability: Unix.

   New in version 3.7: The *ssl_handshake_timeout* and *start_serving*
   parameters.

   Changed in version 3.7: The *path* parameter can now be a *path-
   like object*.


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

class asyncio.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 "open_connection()" and "start_server()" instead.

   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 EOF was received and the internal buffer is empty, return an
      empty "bytes" object.

   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 returns
      partially read data.

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

   coroutine readexactly(n)

      Read exactly *n* bytes.

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

   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.

      If the amount of data read exceeds the configured stream limit,
      a "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
      "IncompleteReadError" exception is raised, and the internal
      buffer is reset.  The "IncompleteReadError.partial" attribute
      may contain a portion of the separator.

      New in version 3.5.2.

   at_eof()

      Return "True" if the buffer is empty and "feed_eof()" was
      called.


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

class asyncio.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 "open_connection()" and "start_server()" instead.

   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 method should be used along with the "drain()" method:

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

   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()

   close()

      The method closes the stream and the underlying socket.

      The method should be used along with the "wait_closed()" method:

         stream.close()
         await stream.wait_closed()

   can_write_eof()

      Return "True" if the underlying transport supports the
      "write_eof()" method, "False" otherwise.

   write_eof()

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

   transport

      Return the underlying asyncio transport.

   get_extra_info(name, default=None)

      Access optional transport information; see
      "BaseTransport.get_extra_info()" for details.

   coroutine 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 "drain()" returns
      immediately.

   is_closing()

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

      New in version 3.7.

   coroutine wait_closed()

      Wait until the stream is closed.

      Should be called after "close()" to wait until the underlying
      connection is closed.

      New in version 3.7.


Examples
========


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

TCP echo client using the "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!'))

See also:

  The TCP echo client protocol example uses the low-level
  "loop.create_connection()" method.


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

TCP echo server using the "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())

See also:

  The TCP echo server protocol example uses the "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


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

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

See also:

  The register an open socket to wait for data using a protocol
  example uses a low-level protocol and the "loop.create_connection()"
  method.

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