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.
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.
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.
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)
The clients so far have been fairly simple. A more complicated example comes with Twisted in the doc/examples directory.
ircLogBot.pyircLogBot.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.
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.