#!/usr/bin/python """ Do a DNS lookup using Twisted's APIs. """ from __future__ import print_function # The Twisted code we'll be using: from twisted.names import client from crochet import setup, wait_for setup() # Crochet layer, wrapping Twisted's DNS library in a blocking call. @wait_for(timeout=5.0) def gethostbyname(name): """Lookup the IP of a given hostname. Unlike socket.gethostbyname() which can take an arbitrary amount of time to finish, this function will raise crochet.TimeoutError if more than 5 seconds elapse without an answer being received. """ d = client.lookupAddress(name) d.addCallback(lambda result: result.payload.dottedQuad()) return d if __name__ == '__main__': # Application code using the public API - notice it works in a normal # blocking manner, with no event loop visible: import sys name = sys.argv ip = gethostbyname(name) print(name, "->", ip)
Monday, April 14, 2014
Crochet is a library for using Twisted more easily from blocking programs and libraries. The latest version, released here at PyCon 2014, includes a much improved API for calling into Twisted from threads. In particular, a timeout is passed in - if it is hit the underlying operation is cancelled, and an exception is raised. Not all APIs in Twisted support cancellation, but for those that do (or APIs you implement) this is a really nice feature. You get high level timeouts (instead of blocking sockets' timeout-per-socket-operation) and automatic cleanup of resources if something takes too long.