PayPal Flask IPN Engine

ipn.py

# -*- coding: utf-8 -*-
"""
    IPN Engine
    ~~~~~~~~~~

    PayPal's Instant Payment Notification (IPN) helps integrate PayPal more
    deeply into your web application.  When activated, PayPal will send a
    POST request to a specified URL when a transaction's status changes.

    The most obvious use is to trigger a state change, enable an account,
    download, or trigger an email when a payment is made.
    
    To validate PayPals POST data, the data must be sent back to PayPal
    in exactly the same order, prefixed with 'cmd=_notify-validate'. If
    valid PayPal responds with 'VERIFIED' if not it returns 'INVALID'.

    Requires: flask, requests

"""

from flask import Flask, request
import requests
from werkzeug.datastructures import ImmutableOrderedMultiDict

app = Flask(__name__)

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

    arg = ''
    #: We use an ImmutableOrderedMultiDict item because it retains the order.
    request.parameter_storage_class = ImmutableOrderedMultiDict()
    values = request.form
    for x, y in values.iteritems():
        arg += "&{x}={y}".format(x=x,y=y)

    validate_url = 'https://www.sandbox.paypal.com' \
                   '/cgi-bin/webscr?cmd=_notify-validate{arg}' \
                   .format(arg=arg)
                  
    print 'Validating IPN using {url}'.format(url=validate_url)

    r = requests.get(validate_url)

    if r.text == 'VERIFIED':
        print "PayPal transaction was verified successfully."
        # Do something with the verified transaction details.
        payer_email =  request.form.get('payer_email')
        print "Pulled {email} from transaction".format(email=payer_email)
    else:
         print 'Paypal IPN string {arg} did not validate'.format(arg=arg)

    return r.text

if __name__ == '__main__':
    app.debug = True
    app.run("127.0.0.1",5000)

Do you know if this works? I'm trying something similar, but the parameters seem to get out of order somehow.

Nevermind. It works. Apparently the Flask Test Client ignores parameter_storage_class, so you can't preserve the parameter ordering in tests.