Twisted includes an event-driven web server. Here's a sample web application; notice how the resource object persists in memory, rather than being recreated on each request:

                  
from twisted.web import server, resource
from twisted.internet import reactor, endpoints

class Counter(resource.Resource):
    isLeaf = True
    numberRequests = 0

    def render_GET(self, request):
        self.numberRequests += 1
        request.setHeader(b"content-type", b"text/plain")
        content = u"I am request #{}\n".format(self.numberRequests)
        return content.encode("ascii")

endpoints.serverFromString(reactor, "tcp:8080").listen(server.Site(Counter()))
reactor.run()
                
                

Learn more about web application development, templates and Twisted' HTTP client.

Here's a simple publish/subscribe server, where clients see all messages posted by other clients:

                  
from twisted.internet import reactor, protocol, endpoints
from twisted.protocols import basic

class PubProtocol(basic.LineReceiver):
    def __init__(self, factory):
        self.factory = factory

    def connectionMade(self):
        self.factory.clients.add(self)

    def connectionLost(self, reason):
        self.factory.clients.remove(self)

    def lineReceived(self, line):
        for c in self.factory.clients:
            source = u"<{}> ".format(self.transport.getHost()).encode("ascii")
            c.sendLine(source + line)

class PubFactory(protocol.Factory):
    def __init__(self):
        self.clients = set()

    def buildProtocol(self, addr):
        return PubProtocol(self)

endpoints.serverFromString(reactor, "tcp:1025").listen(PubFactory())
reactor.run()
                  
                

You can test this out by opening two terminals and doing telnet localhost 1025 in each, then typing things.

Twisted includes a sophisticated IMAP4 client library.

                  
import sys

from twisted.internet import protocol, defer, endpoints, task
from twisted.mail import imap4
from twisted.python import failure


async def main(
    reactor, username="alice", password="secret", strport="tls:example.com:993"
):
    endpoint = endpoints.clientFromString(reactor, strport)
    factory = protocol.Factory.forProtocol(imap4.IMAP4Client)
    try:
        client = await endpoint.connect(factory)
        await client.login(username.encode("utf-8"),
                           password.encode("utf-8"))
        await client.select("INBOX")
        info = await client.fetchEnvelope(imap4.MessageSet(1))
        print("First message subject:", info[1]["ENVELOPE"][1])
    except:
        print("IMAP4 client interaction failed")
        print(failure.Failure().getTraceback())


task.react(lambda *a, **k: defer.ensureDeferred(main(*a, **k)), sys.argv[1:])
                  
                

Give this a try, supplying your IMAP4 username, app password (generate one for gmail, generate one for fastmail), and client endpoint description for your IMAP4 server. You'll see the subject of the first message in your mailbox printed.

See the TwistedMail documentation for more information.

Twisted includes an SSH client & server, "conch" (i.e.: the Twisted Shell).

                  
import sys, os

from twisted.internet import protocol, defer, endpoints, task
from twisted.conch.endpoints import SSHCommandClientEndpoint

async def main(reactor, username="alice", sshhost="example.com", portno="22"):
    envAgent = endpoints.UNIXClientEndpoint(reactor, os.environ["SSH_AUTH_SOCK"])
    endpoint = SSHCommandClientEndpoint.newConnection(
        reactor, "echo 'hello world'", username, sshhost,
        int(portno), agentEndpoint=envAgent,
    )

    class ShowOutput(protocol.Protocol):
        received = b""
        def dataReceived(self, data):
            self.received += data
        def connectionLost(self, reason):
            finished.callback(self.received)

    finished = defer.Deferred()
    factory = protocol.Factory.forProtocol(ShowOutput)
    await endpoint.connect(factory)
    print("SSH response:", await finished)

task.react(lambda *a, **k: defer.ensureDeferred(main(*a, **k)), sys.argv[1:])
                  
                

You can use this client to run "hello world" on any SSH server that your local SSH agent can authenticate to, if you pass your username, host name, and optionally port number on the command line.