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!'))

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.

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.

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().

close(): Close the stream.

is_closing(): Return True if the stream is closed or in the process of being 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!'))

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())

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())

Streams — Python documentation

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