Simple function to do an HTTP POST using Twisted Agent

httpRequest.py

from twisted.web.iweb import IBodyProducer
from twisted.internet import defer
from twisted.web.client import Agent
from twisted.web.http_headers import Headers
import urllib

class StringProducer(object):
    implements(IBodyProducer)

    def __init__(self, body):
        self.body = body
        self.length = len(body)

    def startProducing(self, consumer):
        consumer.write(self.body)
        return succeed(None)

    def pauseProducing(self):
        pass

    def stopProducing(self):
        pass

def httpRequest(url, values={}, headers={}, method='POST'):
    # Construct an Agent.
    agent = Agent(reactor)
    data = urllib.urlencode(values)

    d = agent.request(method,
                      url,
                      Headers(headers),
                      StringProducer(data) if data else None)

    def handle_response(response):
        if response.code == 204:
            d = defer.succeed('')
        else:
            class SimpleReceiver(protocol.Protocol):
                def __init__(s, d):
                    s.buf = ''; s.d = d
                def dataReceived(s, data):
                    s.buf += data
                def connectionLost(s, reason):
                    # TODO: test if reason is twisted.web.client.ResponseDone, if not, do an errback
                    s.d.callback(s.buf)

            d = defer.Deferred()
            response.deliverBody(SimpleReceiver(d))
        return d

    d.addCallback(handle_response)
    return d

# Sample usage:

d = httpRequest(
    "http://...",                            
    {   
        'query_arg': 'value',          
    },
    headers={'Content-Type': ['application/x-www-form-urlencoded']}                      
)

Missing import: from zope.interface import implements

Also line 16 should be

return defer.succeed(None)

Also missing from twisted.internet import reactor and from twisted.internet import protocol

Also default mutable parameters. It would be better to use values=None, headers=None and after that set values/headers ={} if values/headers is None

line 24

def httpRequest(url, values={}, headers={}, method='POST'):

avoid, it's evil ! Python dictionaries are references not a full copies of dictionary objects so you can see very unexpected side effects of such a use cases.