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.

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.

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().

バージョン 3.7 で追加: 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().

バージョン 3.7 で追加: 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.

バージョン 3.7 で追加: The ssl_handshake_timeout parameter.

バージョン 3.7 で変更: 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.

バージョン 3.7 で追加: The ssl_handshake_timeout and start_serving parameters.

バージョン 3.7 で変更: 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)

n バイト読み込みます。n が指定されないか -1 が指定されていた場合 EOF になるまで読み込み、全データを返します。

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

coroutine readline()

1 行読み込みます。 "行" とは、\n で終了するバイト列のシーケンスです。

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.

成功時には、データと区切り文字は内部バッファから削除されます (消費されます)。返されるデータの最後には区切り文字が含まれます。

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.

バージョン 3.5.2 で追加.

at_eof()

バッファーが空で feed_eof() が呼ばれていた場合 True を返します。

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.

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.

バージョン 3.7 で追加.

coroutine wait_closed()

Wait until the stream is closed.

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

バージョン 3.7 で追加.

使用例

ストリームを使った TCP Echo クライアント

asyncio.open_connection() 関数を使った TCP Echo クライアントです:

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!'))

参考

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

ストリームを使った TCP Echo サーバー

asyncio.start_server() 関数を使った TCP Echo サーバーです:

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())

参考

The TCP echo server protocol example uses the loop.create_server() method.

HTTP ヘッダーの取得

コマンドラインから渡された URL の HTTP ヘッダーを問い合わせる簡単な例です:

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))

使い方:

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

または HTTPS を使用:

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

ストリームを使ってデータを待つオープンソケットの登録

open_connection() 関数を使ってソケットがデータを受信するまで待つコルーチンです:

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.