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!'))

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!'))

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())