流
**

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

======================================================================

流是用于处理网络连接的高级 async/await-ready 原语。流允许发送和接收数
据，而不需要使用回调或低级协议和传输。

下面是一个使用 asyncio streams 编写的 TCP echo 客户端示例:

   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!'))

参见下面的 Examples 部分。

-[ Stream 函数 ]-

下面的高级 asyncio 函数可以用来创建和处理流:

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)

   建立网络连接并返回一对 "(reader, writer)" 对象。

   返回的 *reader* 和 *writer* 对象是 "StreamReader" 和 "StreamWriter"
   类的实例。

   *loop* 参数是可选的，当从协程中等待该函数时，总是可以自动确定。

   *limit* 确定返回的 "StreamReader" 实例使用的缓冲区大小限制。默认情
   况下，*limit* 设置为 64 KiB 。

   其余的参数直接传递到 "loop.create_connection()" 。

   3.7 版新加入: *ssl_handshake_timeout* 形参。

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)

   启动套接字服务。

   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* 确定返回的 "StreamReader" 实例使用的缓冲区大小限制。默认情
   况下，*limit* 设置为 64 KiB 。

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

   3.7 版新加入: 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.

   3.7 版新加入: *ssl_handshake_timeout* 形参。

   3.7 版更變: 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)

   启动一个Unix socket服务。

   Similar to "start_server()" but works with Unix sockets.

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

   Availability: Unix.

   3.7 版新加入: The *ssl_handshake_timeout* and *start_serving*
   parameters.

   3.7 版更變: *path* 形参现在可以是 *path-like object* 对象。


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.

      3.5.2 版新加入.

   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.

      3.7 版新加入.

   coroutine wait_closed()

      Wait until the stream is closed.

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

      3.7 版新加入.


示例
====


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!'))

也參考: 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())

也參考: 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))

用法:

   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())

也參考: 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.
