select — I/O multiplexing

import select

Wait for one or more file descriptors (sockets, mostly) to become ready for read / write / error. Backed by lwIP’s select() for sockets.

select.poll() — preferred API

Returns a poll object.

poller = select.poll()
poller.register(sock, select.POLLIN)        # wait for readable
poller.register(sock2, select.POLLIN | select.POLLOUT)

events = poller.poll(timeout_ms=100)
for obj, ev in events:
    if ev & select.POLLIN:
        data = obj.recv(1024)
        ...
    if ev & select.POLLOUT:
        ...

poller.unregister(sock)

Event flags:

• select.POLLIN — readable
• select.POLLOUT — writable
• select.POLLERR — error
• select.POLLHUP — connection hung up
• select.POLLNVAL — invalid fd

timeout_ms of None blocks indefinitely; 0 polls without waiting; positive integer blocks up to that many milliseconds.

select.select(rl, wl, xl, timeout=None) — BSD-style

ready_r, ready_w, ready_x = select.select([sock], [], [], 1.0)
if ready_r:
    data = sock.recv(1024)

timeout is in seconds (float). None blocks.

Example: echo server

import socket, select

srv = socket.socket()
srv.bind(('0.0.0.0', 1234))
srv.listen(5)
poller = select.poll()
poller.register(srv, select.POLLIN)
clients = {}     # fd → socket

while True:
    for fd, ev in poller.poll(1000):
        if fd is srv:
            conn, addr = srv.accept()
            poller.register(conn, select.POLLIN)
            clients[id(conn)] = conn
        elif ev & select.POLLIN:
            data = fd.recv(1024)
            if data:
                fd.send(data)
            else:
                poller.unregister(fd)
                fd.close()
                clients.pop(id(fd), None)

DOS specifics

• Only socket fds work with select — file fds will silently always-report-ready.
• The lwIP timer must be polled regularly (lwip.callback()) for data to actually arrive at sockets. Most users handle this by setting short timeouts on poll()/select() and calling lwip.callback() in the loop.

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Credit: MicroPython select docs (MIT).