## version 1.1, changed according to the suggestions in the comments
from twisted.internet import reactor, defer
from twisted.python import threadable; threadable.init(1)
import sys, time
## the wrong way
def callInThread(func, *args):
"""Takes a blocking function an converts it into a deferred-valued
function running in a separate thread.
"""
de = defer.Deferred()
de.addCallback(func)
reactor.callInThread(de.callback, *args)
return de
deferred = callInThread.__get__ # decorator associated to callInThread
# the right way
from twisted.internet.threads import deferToThread
deferred = deferToThread.__get__
## example code
def print_(result):
print result
def running():
"Prints a few dots on stdout while the reactor is running."
sys.stdout.write("."); sys.stdout.flush()
reactor.callLater(.1, running)
@deferred
def sleep(sec):
"A blocking function magically converted in a non-blocking one."
print ‘start sleep %s‘ % sec
time.sleep(sec)
print ‘\nend sleep %s‘ % sec
return "ok"
if __name__ == "__main__":
sleep(2).addBoth(print_)
reactor.callLater(.1, running)
reactor.callLater(3, reactor.stop)
reactor.run()
Discussion:
How to make blocking code non-blocking is the obvious question for
everybody using Twisted, but the Twisted documentation does not make
easy to find the solution :-(
The trick is to run the blocking function in a separate thread. Here
all the magic is performed by the decorator, ``deferred``, which converts
``sleep``, a blocking function, into a deferred function i.e. a
non-blocking function that returns a deferred object. The code for
``callInThread`` is clear, and the ``.__get__`` trick converts
``callInThread`` in a one-argument function returning a closure,
i.e. an object suitable to be used as a decorator. I have seen this
trick in Alex Martelli‘s lectures at PyCon 2005. If you are confused
by it, you should read Raymond Hettinger essay on descriptors
(http://users./python/download/Descriptor.htm).
In short: every time you have a blocking function in your code, wrap
it with the ``deferred`` decorator and live happy!
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