Using sockets and streams

Networking capabilities are arguably the most important part of any asynchronous library. AnyIO contains its own high level implementation of networking on top of low level primitives offered by each of its supported backends.

Currently AnyIO offers the following networking functionality:

TCP sockets (client + server)
UNIX domain sockets (client + server)
UDP sockets

More exotic forms of networking such as raw sockets and SCTP are currently not supported.

Warning

Unlike the standard BSD sockets interface and most other networking libraries, AnyIO (from 2.0 onwards) signals the end of any stream by raising the EndOfStream exception instead of returning an empty bytes object.

Working with TCP sockets

TCP (Transmission Control Protocol) is the most commonly used protocol on the Internet. It allows one to connect to a port on a remote host and send and receive data in a reliable manner.

To connect to a listening TCP socket somewhere, you can use connect_tcp():

from anyio import connect_tcp, run


async def main():
    async with await connect_tcp('hostname', 1234) as client:
        await client.send(b'Client\n')
        response = await client.receive()
        print(response)

run(main)

As a convenience, you can also use connect_tcp() to establish a TLS session with the peer after connection, by passing tls=True or by passing a nonempty value for either ssl_context or tls_hostname.

To receive incoming TCP connections, you first create a TCP listener with create_tcp_listener() and call serve() on it:

from anyio import create_tcp_listener, run


async def handle(client):
    async with client:
        name = await client.receive(1024)
        await client.send(b'Hello, %s\n' % name)


async def main():
    listener = await create_tcp_listener(local_port=1234)
    await listener.serve(handle)

run(main)

See the section on TLS streams for more information.

Working with UNIX sockets

UNIX domain sockets are a form of interprocess communication on UNIX-like operating systems. They cannot be used to connect to remote hosts and do not work on Windows.

The API for UNIX domain sockets is much like the one for TCP sockets, except that instead of host/port combinations, you use file system paths.

This is what the client from the TCP example looks like when converted to use UNIX sockets:

from anyio import connect_unix, run


async def main():
    async with await connect_unix('/tmp/mysock') as client:
        await client.send(b'Client\n')
        response = await client.receive(1024)
        print(response)

run(main)

And the listener:

from anyio import create_unix_listener, run


async def handle(client):
    async with client:
        name = await client.receive(1024)
        await client.send(b'Hello, %s\n' % name)


async def main():
    listener = await create_unix_listener('/tmp/mysock')
    await listener.serve(handle)

run(main)

Note

The UNIX socket listener does not remove the socket it creates, so you may need to delete them manually.

Sending and receiving file descriptors

UNIX sockets can be used to pass open file descriptors (sockets and files) to another process. The receiving end can then use either os.fdopen() or socket.socket() to get a usable file or socket object, respectively.

The following is an example where a client connects to a UNIX socket server and receives the descriptor of a file opened on the server, reads the contents of the file and then prints them on standard output.

Client:

import os

from anyio import connect_unix, run


async def main():
    async with await connect_unix('/tmp/mysock') as client:
        _, fds = await client.receive_fds(0, 1)
        with os.fdopen(fds[0]) as file:
            print(file.read())

run(main)

Server:

from pathlib import Path

from anyio import create_unix_listener, run


async def handle(client):
    async with client:
        with path.open('r') as file:
            await client.send_fds(b'this message is ignored', [file])


async def main():
    listener = await create_unix_listener('/tmp/mysock')
    await listener.serve(handle)

path = Path('/tmp/examplefile')
path.write_text('Test file')
run(main)

Working with UDP sockets

UDP (User Datagram Protocol) is a way of sending packets over the network without features like connections, retries or error correction.

For example, if you wanted to create a UDP “hello” service that just reads a packet and then sends a packet to the sender with the contents prepended with “Hello, “, you would do this:

import socket

from anyio import create_udp_socket, run


async def main():
    async with await create_udp_socket(family=socket.AF_INET, local_port=1234) as udp:
        async for packet, (host, port) in udp:
            await udp.sendto(b'Hello, ' + packet, host, port)

run(main)

Note

If you are testing on your local machine or don’t know which family socket to use, it is a good idea to replace family=socket.AF_INET by local_host='localhost' in the previous example.

If your use case involves sending lots of packets to a single destination, you can still “connect” your UDP socket to a specific host and port to avoid having to pass the address and port every time you send data to the peer:

from anyio import create_connected_udp_socket, run


async def main():
    async with await create_connected_udp_socket(
            remote_host='hostname', remote_port=1234) as udp:
        await udp.send(b'Hi there!\n')

run(main)