app.yaml

application: your-app-id
version: 1
runtime: python27
api_version: 1
threadsafe: true

handlers:
- url: /.*
  script: demo.application

demo.py

import webapp2
import time

from google.appengine.ext import db


class MyData(db.Model):
    val = db.IntegerProperty()
    modified = db.DateTimeProperty(auto_now=True)


class MainPage(webapp2.RequestHandler):

    def get(self):
        self.response.headers['Content-Type'] = 'text/plain'
        obj = MyData()
        obj.val = 1
        key = obj.put()
        self.response.write('modified 1: %s\n' % obj.modified)
        # time.sleep(1)
        obj = db.get(key)
        obj.__class__.modified.auto_now = False
        self.response.write('value is %s\n' % obj.val)
        obj.val = 2
        obj.put()
        self.response.write('modified 2: %s\n' % obj.modified)
        obj.__class__.modified.auto_now = True


application = webapp2.WSGIApplication([
    ('/', MainPage),
], debug=True)