# 流

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())

        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 新版功能: 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。

  当一个新的客户端连接被建立时，回调函数 client_connected_cb 会被调用。该函数会接收到一对参数 `(reader, writer)` ，reader是类 `StreamReader` 的实例，而writer是类 `StreamWriter` 的实例。

  client_connected_cb 即可以是普通的可调用对象也可以是一个 协程函数; 如果它是一个协程函数，它将自动作为 `Task` 被调度。

  loop 参数是可选的。当在一个协程中await该方法时，该参数始终可以自动确定。

  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 。

  余下的参数将会直接传递给 `loop.create_server()`.

  3.7 新版功能: The ssl_handshake_timeout and start_serving parameters.

Unix 套接字

coroutine `asyncio.``open_unix_connection`(path=None, *, loop=None, limit=None, ssl=None, sock=None, server_hostname=None, ssl_handshake_timeout=None)

  建立一个 Unix 套接字连接并返回 `(reader, writer)` 这对返回值。

  与 `open_connection()` 相似，但是操作在 Unix 套接字上

  请看文档 `loop.create_unix_connection()`.

  可用性: Unix。

  3.7 新版功能: ssl_handshake_timeout 形参。

  在 3.7 版更改: path 现在是一个 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。

  与 `start_server()` 相似，但是是在 Unix 套接字上的操作。

  请看文档 `loop.create_unix_server()`.

  可用性: Unix。

  3.7 新版功能: The ssl_handshake_timeout and start_serving parameters.

  在 3.7 版更改: path 形参现在可以是 path-like object 对象。

## StreamReader

class `asyncio.``StreamReader`

  这个类表示一个提供api来从IO流中读取数据的读取器对象。

  不推荐直接实例化 StreamReader 对象，建议使用 `open_connection()` 和 `start_server()` 来获取 StreamReader 实例。

  coroutine `read`(n=-1)

    读取 n 个byte. 如果没有设置 n , 则自动置为 `-1` ，读至 EOF 并返回所有读取的byte。

    如果读到EOF，且内部缓冲区为空，则返回一个空的 `bytes` 对象。

  coroutine `readline`()

    读取一行，其中“行”指的是以 `\n` 结尾的字节序列。

    如果读到EOF而没有找到 `\n` ，该方法返回部分读取的数据。

    如果读到EOF，且内部缓冲区为空，则返回一个空的 `bytes` 对象。

  coroutine `readexactly`(n)

    精准读取 n 个 bytes，不能超过也不能少于。

    如果在读取完 n 个byte之前读取到EOF，则会抛出 `IncompleteReadError` 异常。使用 `IncompleteReadError.partial` 属性来获取到达流结束之前读取的 bytes 字符串。

  coroutine `readuntil`(separator=b'\n')

    从流中读取数据直至遇到 分隔符

    成功后，数据和指定的separator将从内部缓冲区中删除(或者说被消费掉)。返回的数据将包括在末尾的指定separator。

    如果读取的数据量超过了配置的流限制，将引发 `LimitOverrunError` 异常，数据将留在内部缓冲区中并可以再次读取。

    如果在找到完整的separator之前到达EOF，则会引发 `IncompleteReadError` 异常，并重置内部缓冲区。 `IncompleteReadError.partial` 属性可能包含指定separator的一部分。

    3.5.2 新版功能.

  `at_eof`()

    如果缓冲区为空并且 `feed_eof()` 被调用，则返回 `True` 。

## StreamWriter

class `asyncio.``StreamWriter`

  这个类表示一个写入器对象，该对象提供api以便于写数据至IO流中。

  It is not recommended to instantiate StreamWriter objects directly; use `open_connection()` and `start_server()` instead.

  `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.

  `write`(data)

    Write data to the stream.

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

  `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 `drain()`.

  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.

  `close`()

    Close the stream.

  `is_closing`()

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

    3.7 新版功能.

  coroutine `wait_closed`()

    Wait until the stream is closed.

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

    3.7 新版功能.

## 例子

### 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!'))

参见

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())

参见

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())

参见

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.