Overview
Twisted is a framework designed to be very flexible, and let you write powerful clients. The cost of this flexibility is a few layers in the way to writing your client. This document covers creating clients that can be used for TCP, SSL and Unix sockets, UDP is covered in a different document.
At the base, the place where you actually implement the protocol parsing
and handling, is the Protocol class. This class will usually be decended
from twisted.internet.protocol.Protocol
. Most
protocol handlers inherit either from this class or from one of its
convenience children. An instance of the protocol class will be
instantiated when you connect to the server, and will go away when the
connection is finished. This means that persistent configuration is not
saved in the Protocol.
The persistent configuration is kept in a Factory class, which usually
inherits from twisted.internet.protocol.ClientFactory
. The default
factory class just instantiate the Protocol, and then sets on it an
attribute called factory
which points to itself. This let
the Protocol access, and possibly modify, the persistent
configuration.
Protocol
As mentioned above, this, and auxiliary classes and functions, is where most of the code is. A Twisted protocol handles data in an asynchronous manner. What this means is that the protocol never waits for an event, but rather responds to events as they arrive from the network.
Here is a simple example:
from twisted.internet.protocol import Protocol from sys import stdout class Echo(Protocol): def dataReceived(self, data): stdout.write(data)
This is one of the simplest protocols. It simply writes to standard output whatever it reads from the connection. There are many events it does not respond to. Here is an example of a Protocol responding to another event.
from twisted.internet.protocol import Protocol class WelcomeMessage(Protocol): def connectionMade(self): self.transport.write("Hello server, I am the client!\r\n") self.transport.loseConnection()
This protocol connects to the server, sends it a welcome message, and then terminates the connection.
The connectionMade event is usually where set up of the Protocol object happens, as well as any initial greetings (as in the WelcomeMessage protocol above). Any tearing down of Protocol-specific objects is done in connectionLost.
Simple, single-use clients
In many cases, the protocol only needs to connect to the server once,
and the code just wants to get a connected instance of the protocol. In
those cases twisted.internet.protocol.ClientCreator
provides the
appropriate API.
from twisted.internet import reactor from twisted.internet.protocol import Protocol, ClientCreator class Greeter(Protocol): def sendMessage(self, msg): self.transport.write("MESSAGE %s\n" % msg) def gotProtocol(p): p.sendMessage("Hello") reactor.callLater(1, p.sendMessage, "This is sent in a second") reactor.callLater(2, p.transport.loseConnection) c = ClientCreator(reactor, Greeter) c.connectTCP("localhost", 1234).addCallback(gotProtocol)
ClientFactory
We use reactor.connect* and a ClientFactory. The ClientFactory is in charge of creating the Protocol, and also receives events relating to the connection state. This allows it to do things like reconnect on the event of a connection error. Here is an example of a simple ClientFactory that uses the Echo protocol (above) and also prints what state the connection is in.
from twisted.internet.protocol import Protocol, ClientFactory from sys import stdout class Echo(Protocol): def dataReceived(self, data): stdout.write(data) class EchoClientFactory(ClientFactory): def startedConnecting(self, connector): print 'Started to connect.' def buildProtocol(self, addr): print 'Connected.' return Echo() def clientConnectionLost(self, connector, reason): print 'Lost connection. Reason:', reason def clientConnectionFailed(self, connector, reason): print 'Connection failed. Reason:', reason
To connect this EchoClientFactory to a server, you could use this code:
from twisted.internet import reactor reactor.connectTCP(host, port, EchoClientFactory()) reactor.run()
Note that clientConnectionFailed
is called when a connection could not be established, and that clientConnectionLost
is called when a connection was made and then disconnected.
Reconnection
Many times, the connection of a client will be lost unintentionally due
to network errors. One way to reconnect after a disconnection would be to
call connector.connect()
when the
connection is lost:
from twisted.internet.protocol import ClientFactory class EchoClientFactory(ClientFactory): def clientConnectionLost(self, connector, reason): connector.connect()
The connector passed as the first argument is the interface between a
connection and a protocol. When the connection fails and the factory
receives the clientConnectionLost event, the factory can call connector.connect()
to start the connection over
again from scratch.
However, most programs that want this functionality should implement ReconnectingClientFactory
instead,
which tries to reconnect if a connection is lost or fails, and which
exponentially delays repeated reconnect attempts.
Here is the Echo protocol implemented with a ReconnectingClientFactory:
from twisted.internet.protocol import Protocol, ReconnectingClientFactory from sys import stdout class Echo(Protocol): def dataReceived(self, data): stdout.write(data) class EchoClientFactory(ReconnectingClientFactory): def startedConnecting(self, connector): print 'Started to connect.' def buildProtocol(self, addr): print 'Connected.' print 'Resetting reconnection delay' self.resetDelay() return Echo() def clientConnectionLost(self, connector, reason): print 'Lost connection. Reason:', reason ReconnectingClientFactory.clientConnectionLost(self, connector, reason) def clientConnectionFailed(self, connector, reason): print 'Connection failed. Reason:', reason ReconnectingClientFactory.clientConnectionFailed(self, connector, reason)
A Higher-Level Example: ircLogBot
Overview of ircLogBot
The clients so far have been fairly simple. A more complicated example comes with Twisted Words in the doc/examples directory.
# twisted imports from twisted.words.protocols import irc from twisted.internet import reactor, protocol from twisted.python import log # system imports import time, sys class MessageLogger: """ An independent logger class (because separation of application and protocol logic is a good thing). """ def __init__(self, file): self.file = file def log(self, message): """Write a message to the file.""" timestamp = time.strftime("[%H:%M:%S]", time.localtime(time.time())) self.file.write('%s %s\n' % (timestamp, message)) self.file.flush() def close(self): self.file.close() class LogBot(irc.IRCClient): """A logging IRC bot.""" nickname = "twistedbot" def connectionMade(self): irc.IRCClient.connectionMade(self) self.logger = MessageLogger(open(self.factory.filename, "a")) self.logger.log("[connected at %s]" % time.asctime(time.localtime(time.time()))) def connectionLost(self, reason): irc.IRCClient.connectionLost(self, reason) self.logger.log("[disconnected at %s]" % time.asctime(time.localtime(time.time()))) self.logger.close() # callbacks for events def signedOn(self): """Called when bot has succesfully signed on to server.""" self.join(self.factory.channel) def joined(self, channel): """This will get called when the bot joins the channel.""" self.logger.log("[I have joined %s]" % channel) def privmsg(self, user, channel, msg): """This will get called when the bot receives a message.""" user = user.split('!', 1)[0] self.logger.log("<%s> %s" % (user, msg)) # Check to see if they're sending me a private message if channel == self.nickname: msg = "It isn't nice to whisper! Play nice with the group." self.msg(user, msg) return # Otherwise check to see if it is a message directed at me if msg.startswith(self.nickname + ":"): msg = "%s: I am a log bot" % user self.msg(channel, msg) self.logger.log("<%s> %s" % (self.nickname, msg)) def action(self, user, channel, msg): """This will get called when the bot sees someone do an action.""" user = user.split('!', 1)[0] self.logger.log("* %s %s" % (user, msg)) # irc callbacks def irc_NICK(self, prefix, params): """Called when an IRC user changes their nickname.""" old_nick = prefix.split('!')[0] new_nick = params[0] self.logger.log("%s is now known as %s" % (old_nick, new_nick)) class LogBotFactory(protocol.ClientFactory): """A factory for LogBots. A new protocol instance will be created each time we connect to the server. """ # the class of the protocol to build when new connection is made protocol = LogBot def __init__(self, channel, filename): self.channel = channel self.filename = filename def clientConnectionLost(self, connector, reason): """If we get disconnected, reconnect to server.""" connector.connect() def clientConnectionFailed(self, connector, reason): print "connection failed:", reason reactor.stop() if __name__ == '__main__': # initialize logging log.startLogging(sys.stdout) # create factory protocol and application f = LogBotFactory(sys.argv[1], sys.argv[2]) # connect factory to this host and port reactor.connectTCP("irc.freenode.net", 6667, f) # run bot reactor.run()
ircLogBot.py connects to an IRC server, joins a channel, and logs all traffic on it to a file. It demonstrates some of the connection-level logic of reconnecting on a lost connection, as well as storing persistent data in the Factory.
Persistent Data in the Factory
Since the Protocol instance is recreated each time the connection is made, the client needs some way to keep track of data that should be persisted. In the case of the logging bot, it needs to know which channel it is logging, and where to log it to.
from twisted.internet import protocol from twisted.protocols import irc class LogBot(irc.IRCClient): def connectionMade(self): irc.IRCClient.connectionMade(self) self.logger = MessageLogger(open(self.factory.filename, "a")) self.logger.log("[connected at %s]" % time.asctime(time.localtime(time.time()))) def signedOn(self): self.join(self.factory.channel) class LogBotFactory(protocol.ClientFactory): protocol = LogBot def __init__(self, channel, filename): self.channel = channel self.filename = filename
When the protocol is created, it gets a reference to the factory as self.factory. It can then access attributes of the factory in its logic. In the case of LogBot, it opens the file and connects to the channel stored in the factory.
Further Reading
The Protocol
class used throughout this document is a base implementation of IProtocol
used in
most Twisted applications for convenience. To learn about the
completeIProtocol
interface, see the API documentation for
IProtocol
.
The transport
attribute used in some examples in this
document provides the
ITCPTransport
interface. To learn about the complete interface, see
the API documentation for ITCPTransport
.
Interface classes are a way of specifying what methods and attributes an object has and how they behave. See the Components: Interfaces and Adapters document.