Code sample included with "Burp Extensions in Python & Pentesting Custom Web Services" at http://labs.neohapsis.com/

BurpPluginDemo.py

# These are java classes, being imported using python syntax (Jython magic)
from burp import IBurpExtender
from burp import IHttpListener

# These are plain old python modules, from the standard library 
# (or from the "Folder for loading modules" in Burp>Extender>Options)
from datetime import datetime

class BurpExtender(IBurpExtender, IHttpListener):
        
    def registerExtenderCallbacks(self, callbacks):
        self._callbacks = callbacks
        self._helpers = callbacks.getHelpers()
        callbacks.setExtensionName("Burp Plugin Python Demo")
        callbacks.registerHttpListener(self)
        return

    def processHttpMessage(self, toolFlag, messageIsRequest, currentRequest):
        # only process requests
        if not messageIsRequest:
            return
        
        requestInfo = self._helpers.analyzeRequest(currentRequest)
        timestamp = datetime.now()        
        print "Intercepting message at:", timestamp.isoformat()
        
        headers = requestInfo.getHeaders()
        newHeaders = list(headers) #it's a Java arraylist; get a python list
        newHeaders.append("Timestamp: " + timestamp.isoformat())
        
        bodyBytes = currentRequest.getRequest()[requestInfo.getBodyOffset():]
        bodyStr = self._helpers.bytesToString(bodyBytes)
        newMsgBody = bodyStr + timestamp.isoformat()
        
        newMessage = self._helpers.buildHttpMessage(newHeaders, newMsgBody)
        print "Sending modified message:"
        print "----------------------------------------------"
        print self._helpers.bytesToString(newMessage)
        print "----------------------------------------------\n\n"
        currentRequest.setRequest(newMessage)
        return