Event-Driven Python: Mastering Twisted and Deferreds
Threads are a trap. They're heavy, they have race conditions, and they don't scale. If you want to build a high-performance chat server or a web crawler in Python, you need Twisted. It's the "Matrix" of networking frameworks: it's everywhere once you know how to see it.
The Reactor Loop
In Twisted, everything revolves around the reactor. You don't call recv() and wait for data. Instead, you register a callback that gets triggered when data arrives.
from twisted.internet import protocol, reactor
class Echo(protocol.Protocol):
def dataReceived(self, data):
self.transport.write(data)
class EchoFactory(protocol.Factory):
def buildProtocol(self, addr):
return Echo()
reactor.listenTCP(8000, EchoFactory())
reactor.run()
Understanding Deferreds
The hardest part of Twisted for newcomers is the Deferred. It's a promise of a result that hasn't arrived yet. Instead of waiting for a database query to finish, you get a Deferred object and attach addCallback and addErrback to it.
def printResult(result):
print "The data arrived:", result
def handleError(failure):
print "Something went wrong:", failure
d = getRemoteData() # Returns a Deferred
d.addCallback(printResult)
d.addErrback(handleError)
Why Bother?
It feels complicated at first, but the benefit is massive. You can handle 10,000 concurrent connections on a single CPU because the process never sleeps. While one connection is waiting for a disk read, the reactor is already processing 50 other network packets.
Twisted is the future of Python networking. It's used by huge projects like Buildbot and even Apple's Calendar Server. If you're still using SocketServer and threading, you're living in the past.
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