PayPal IPN message verification. Example is in Flask, but it should be easy to move it to other framework. It is encoding proof :), so when paypal send you a message with encoded characters it will handle it smoothly. If you want to move it to different framework remember that 'request.data' is a string in Flask not a unicode.

PayPal IPN verification, Flask, Python

import urllib
import urlparse

from flask import request

from your_app import app

# set it to True in production mode
app.config['PAYPAL_LIVE'] = False


PP_SCR_LIVE_URL = 'https://www.paypal.com/cgi-bin/webscr'
PP_SCR_SANDBOX_URL = 'https://www.sandbox.paypal.com/cgi-bin/webscr'

PP_SCR_URL = PP_SCR_LIVE_URL if app.config['PAYPAL_LIVE'] else PP_SCR_SANDBOX_URL


def verify_ipn_message(data):
    verify_string = 'cmd=_notify-validate&%s' % data
    response = urllib.urlopen(PP_SCR_URL, data=verify_string)
    return response.read() == 'VERIFIED', verify_string


@app.route('/paypal/ipn', methods=['POST'])
def ipn():

    verified, verify_string = verify_ipn_message(request.data)

    if verified:
        # Do something with received message if it is valid 
        print "PayPal transaction was verified successfully."
        
        # eg. parse it
        data = urlparse.parse_qs(verify_string)
        # or create a dict:
        # data = dict(urlparse.parse_qsl(verify_string))
        print "IPN message: ", data
        
        return "Paypal IPN message verified.", 200
    else:
        # Do something if it is invalid
        return "Paypal IPN message could not be verified.", 403