This is the fourth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this section of the tutorial, we’ll move our code to a component architecture so that adding new features is trivial. See Interfaces and Adapters for a more complete discussion of components.
In the last version, the service class was three times longer than any other
class, and was hard to understand. This was because it turned out to have
multiple responsibilities. It had to know how to access user information, by
rereading the file every half minute, but also how to display itself in a myriad
of protocols. Here, we used the component-based architecture that Twisted
provides to achieve a separation of concerns. All the service is responsible
for, now, is supporting getUser /getUsers . It declares
its support via the zope.interface.implementer decorator. Then, adapters
are used to make this service look like an appropriate class for various things:
for supplying a finger factory to TCPServer , for supplying a
resource to site’s constructor, and to provide an IRC client factory
for TCPClient . All the adapters use are the methods
in FingerService they are declared to use:getUser /getUsers . We could, of course, skip the
interfaces and let the configuration code use things
like FingerFactoryFromService(f) directly. However, using
interfaces provides the same flexibility inheritance gives: future subclasses
can override the adapters.
# Do everything properly, and componentize
import cgi
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
from twisted.web import resource, server, static, xmlrpc
from twisted.words.protocols import irc
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 IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return b"Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value + b"\r\n")
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning L{bytes}
"""
def buildProtocol(addr):
"""
Return a protocol returning L{bytes}
"""
@implementer(IFingerFactory)
class FingerFactoryFromService(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning L{bytes}.
"""
def buildProtocol(addr):
"""
Return a protocol returning L{bytes}.
"""
@implementer(IFingerSetterFactory)
class FingerSetterFactoryFromService(protocol.ServerFactory):
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(
FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory
)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split("!")[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg.encode("ascii"))
d.addErrback(catchError)
d.addCallback(lambda m: f"Status of {msg}: {m}")
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
@implementer(IIRCClientFactory)
class IRCClientFactoryFromService(protocol.ClientFactory):
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(
IRCClientFactoryFromService, IFingerService, IIRCClientFactory
)
@implementer(resource.IResource)
class UserStatusTree(resource.Resource):
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
self.putChild("RPC2", UserStatusXR(self.service))
def render_GET(self, request):
d = self.service.getUsers()
def formatUsers(users):
l = [f'<li><a href="{user}">{user}</a></li>' for user in users]
return "<ul>" + "".join(l) + "</ul>"
d.addCallback(formatUsers)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
def getChild(self, path, request):
if path == "":
return UserStatusTree(self.service)
else:
return UserStatus(path, self.service)
components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(cgi.escape)
d.addCallback(lambda m: "<h1>%s</h1>" % self.user + "<p>%s</p>" % m)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
@implementer(IFingerService)
class FingerService(service.Service):
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
with open(self.filename, "rb") as f:
for line in f:
user, status = line.split(b":", 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, b"No such user"))
def getUsers(self):
return defer.succeed(list(self.users.keys()))
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application("finger", uid=1, gid=1)
f = FingerService("/etc/users")
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
strports.service("tcp:79", IFingerFactory(f)).setServiceParent(serviceCollection)
strports.service("tcp:8000", server.Site(resource.IResource(f))).setServiceParent(
serviceCollection
)
i = IIRCClientFactory(f)
i.nickname = "fingerbot"
internet.ClientService(
endpoints.clientFromString(reactor, "tcp:irc.freenode.org:6667"), i
).setServiceParent(serviceCollection)
Readable – each class is short
Maintainable – each class knows only about interfaces
Dependencies between code parts are minimized
Example: writing a new IFingerService is easy
class IFingerSetterService(Interface):
def setUser(user, status):
"""Set the user's status to something"""
# Advantages of latest version
@implementer(IFingerService, IFingerSetterService)
class MemoryFingerService(service.Service):
def __init__(self, users):
self.users = users
def getUser(self, user):
return defer.succeed(self.users.get(user, b"No such user"))
def getUsers(self):
return defer.succeed(list(self.users.keys()))
def setUser(self, user, status):
self.users[user] = status
f = MemoryFingerService({b"moshez": b"Happy and well"})
serviceCollection = service.IServiceCollection(application)
strports.service(
"tcp:1079:interface=127.0.0.1", IFingerSetterFactory(f)
).setServiceParent(serviceCollection)
Full source code here:
# Do everything properly, and componentize
import cgi
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
from twisted.web import resource, server, static, xmlrpc
from twisted.words.protocols import irc
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 IFingerSetterService(Interface):
def setUser(user, status):
"""Set the user's status to something"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value + b"\r\n")
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""Return a deferred returning L{bytes}"""
def buildProtocol(addr):
"""Return a protocol returning L{bytes}"""
@implementer(IFingerFactory)
class FingerFactoryFromService(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""Return a deferred returning L{bytes}"""
def buildProtocol(addr):
"""Return a protocol returning L{bytes}"""
@implementer(IFingerSetterFactory)
class FingerSetterFactoryFromService(protocol.ServerFactory):
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(
FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory
)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split("!")[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: f"Status of {msg}: {m}")
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""Return a deferred returning a string"""
def buildProtocol(addr):
"""Return a protocol"""
@implementer(IIRCClientFactory)
class IRCClientFactoryFromService(protocol.ClientFactory):
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(
IRCClientFactoryFromService, IFingerService, IIRCClientFactory
)
@implementer(resource.IResource)
class UserStatusTree(resource.Resource):
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
self.putChild("RPC2", UserStatusXR(self.service))
def render_GET(self, request):
d = self.service.getUsers()
def formatUsers(users):
l = [f'<li><a href="{user}">{user}</a></li>' for user in users]
return "<ul>" + "".join(l) + "</ul>"
d.addCallback(formatUsers)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
def getChild(self, path, request):
if path == "":
return UserStatusTree(self.service)
else:
return UserStatus(path, self.service)
components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(cgi.escape)
d.addCallback(lambda m: "<h1>%s</h1>" % self.user + "<p>%s</p>" % m)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
@implementer(IFingerService, IFingerSetterService)
class MemoryFingerService(service.Service):
def __init__(self, users):
self.users = users
def getUser(self, user):
return defer.succeed(self.users.get(user, b"No such user"))
def getUsers(self):
return defer.succeed(list(self.users.keys()))
def setUser(self, user, status):
self.users[user] = status
application = service.Application("finger", uid=1, gid=1)
f = MemoryFingerService({b"moshez": b"Happy and well"})
serviceCollection = service.IServiceCollection(application)
strports.service("tcp:79", IFingerFactory(f)).setServiceParent(serviceCollection)
strports.service("tcp:8000", server.Site(resource.IResource(f))).setServiceParent(
serviceCollection
)
i = IIRCClientFactory(f)
i.nickname = "fingerbot"
internet.ClientService(
endpoints.clientFromString(reactor, "tcp:irc.freenode.org:6667"), i
).setServiceParent(serviceCollection)
strports.service(
"tcp:1079:interface=127.0.0.1", IFingerSetterFactory(f)
).setServiceParent(serviceCollection)
At last, an example of aspect-oriented programming that isn’t about logging or timing. This code is actually useful! Watch how aspect-oriented programming helps you write less code and have fewer dependencies!