The Evolution of Finger: making a finger library


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

In this part, we separate the application code that launches a finger service from the library code which defines a finger service, placing the application in a Twisted Application Configuration (.tac) file. We also move configuration (such as HTML templates) into separate files. Configuration and deployment with .tac and twistd are introduced in Using the Twisted Application Framework .


Now this code, while quite modular and well-designed, isn’t properly organized. Everything above the application= belongs in a module, and the HTML templates all belong in separate files.

We can use the templateFile and templateDirectory attributes to indicate what HTML template file to use for each Page, and where to look for it.


# organized-finger.tac
# eg:  twistd -ny organized-finger.tac

import finger

from twisted.internet import protocol, reactor, defer, endpoints
from twisted.spread import pb
from twisted.web import resource, server
from twisted.application import internet, service, strports
from twisted.python import log

application = service.Application('finger', uid=1, gid=1)
f = finger.FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
strports.service("tcp:79", finger.IFingerFactory(f)

site = server.Site(resource.IResource(f))
strports.service("tcp:8000", site,

strports.service("ssl:port=443:certKey=cert.pem:privateKey=key.pem", site

i = finger.IIRCClientFactory(f)
i.nickname = 'fingerbot'
    endpoints.clientFromString(reactor, ""),

strports.service("tcp:8889", pb.PBServerFactory(finger.IPerspectiveFinger(f))

Note that our program is now quite separated. We have:

  • Code (in the module)
  • Configuration (file above)
  • Presentation (templates)
  • Content (/etc/users )
  • Deployment (twistd)

Prototypes don’t need this level of separation, so our earlier examples all bunched together. However, real applications do. Thankfully, if we write our code correctly, it is easy to achieve a good separation of parts.

Easy Configuration

We can also supply easy configuration for common cases with a makeService method that will also help build .tac files later:

# Easy configuration
# makeService from finger module

def makeService(config):
    # finger on port 79
    s = service.MultiService()
    f = FingerService(config['file'])
    h = strports.service("tcp:79", IFingerFactory(f))

    # website on port 8000
    r = resource.IResource(f)
    r.templateDirectory = config['templates']
    site = server.Site(r)
    j = strports.service("tcp:8000", site)

    # ssl on port 443
    if config.get('ssl'):
        k = strports.service(
            "ssl:port=443:certKey=cert.pem:privateKey=key.pem", site

    # irc fingerbot
    if 'ircnick' in config:
        i = IIRCClientFactory(f)
        i.nickname = config['ircnick']
        ircserver = config['ircserver']
        b = internet.ClientService(
            endpoints.HostnameEndpoint(reactor, ircserver, 6667), i

    # Pespective Broker on port 8889
    if 'pbport' in config:
        m = internet.StreamServerEndpointService(
            endpoints.TCP4ServerEndpoint(reactor, int(config['pbport'])),

    return s

And we can write simpler files now:


# simple-finger.tac
# eg:  twistd -ny simple-finger.tac

from twisted.application import service

import finger

options = { 'file': '/etc/users',
            'templates': '/usr/share/finger/templates',
            'ircnick': 'fingerbot',           
            'ircserver': '',
            'pbport': 8889,
            'ssl': 'ssl=0' }

ser = finger.makeService(options)
application = service.Application('finger', uid=1, gid=1)
% twistd -ny simple-finger.tac

Note: the finger user still has ultimate power: they can use makeService, or they can use the lower-level interface if they have specific needs (maybe an IRC server on some other port? Maybe we want the non-SSL webserver to listen only locally? etc. etc.). This is an important design principle: never force a layer of abstraction; allow usage of layers of abstractions instead.

The pasta theory of design:

Each piece of code interacts with every other piece of code (can be implemented with GOTO, functions, objects).
Code has carefully designed layers. Each layer is, in theory independent. However low-level layers usually cannot be used easily, and high-level layers depend on low-level layers.
Each part of the code is useful by itself. There is a thin layer of interfaces between various parts (the sauce). Each part can be usefully be used elsewhere.

...but sometimes, the user just wants to order “Ravioli”, so one coarse-grain easily definable layer of abstraction on top of it all can be useful.