# The Evolution of Finger: a Twisted finger client

## Introduction

This is the ninth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .

In this part, we develop a client for the finger server: a proxy finger server which forwards requests to another finger server.

## Finger Proxy

Writing new clients with Twisted is much like writing new servers. We implement the protocol, which just gathers up all the data, and give it to the factory. The factory keeps a deferred which is triggered if the connection either fails or succeeds. When we use the client, we first make sure the deferred will never fail, by producing a message in that case. Implementing a wrapper around client which just returns the deferred is a common pattern. While less flexible than using the factory directly, it’s also more convenient.

Additionally, because this code now programmatically receives its host and port, it’s a bit less convenient to use clientFromString. Instead, we move to using the specific endpoint we want. In this case, because we’re connecting as a client over IPv4 using TCP, we want the `TCP4ClientEndpoint`.

`fingerproxy.tac`

    # finger proxy
    from zope.interface import Interface, implementer

    from twisted.application import internet, service, strports
    from twisted.internet import defer, endpoints, protocol, reactor
    from twisted.protocols import basic
    from twisted.python import components

    def catchError(err):
        return "Internal error in server"

    class IFingerService(Interface):
        def getUser(user):
            """Return a deferred returning L{bytes}"""

        def getUsers():
            """Return a deferred returning a L{list} of L{bytes}"""

    class IFingerFactory(Interface):
        def getUser(user):
            """Return a deferred returning L{bytes}"""

        def buildProtocol(addr):
            """Return a protocol returning L{bytes}"""

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            d = self.factory.getUser(user)
            d.addErrback(catchError)

            def writeValue(value):
                self.transport.write(value)
                self.transport.loseConnection()

            d.addCallback(writeValue)

    @implementer(IFingerFactory)
    class FingerFactoryFromService(protocol.ClientFactory):

        protocol = FingerProtocol

        def __init__(self, service):
            self.service = service

        def getUser(self, user):
            return self.service.getUser(user)

    components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory)

    class FingerClient(protocol.Protocol):
        def connectionMade(self):
            self.transport.write(self.factory.user + b"\r\n")
            self.buf = []

        def dataReceived(self, data):
            self.buf.append(data)

        def connectionLost(self, reason):
            self.factory.gotData("".join(self.buf))

    class FingerClientFactory(protocol.ClientFactory):

        protocol = FingerClient

        def __init__(self, user):
            self.user = user
            self.d = defer.Deferred()

        def clientConnectionFailed(self, _, reason):
            self.d.errback(reason)

        def gotData(self, data):
            self.d.callback(data)

    def finger(user, host, port=79):
        f = FingerClientFactory(user)
        endpoint = endpoints.TCP4ClientEndpoint(reactor, host, port)
        endpoint.connect(f)
        return f.d

    @implementer(IFingerService)
    class ProxyFingerService(service.Service):
        def getUser(self, user):
            try:
                user, host = user.split("@", 1)
            except BaseException:
                user = user.strip()
                host = "127.0.0.1"
            ret = finger(user, host)
            ret.addErrback(lambda _: "Could not connect to remote host")
            return ret

        def getUsers(self):
            return defer.succeed([])

    application = service.Application("finger", uid=1, gid=1)
    f = ProxyFingerService()
    strports.service("tcp:7779", IFingerFactory(f)).setServiceParent(
        service.IServiceCollection(application)
    )