title: Appengine: TypeError: 'NoneType' object does not support item assignment
body: <p>Tring to assign a value via constructor. but getting "TypeError: 'NoneType' object does not support item assignment".</p>
  
  <pre><code>import webapp2
  from google.appengine.ext import ndb
  
  class test(ndb.Model):
      status = ndb.StringProperty(default=None)
  
      def __init__(self):
           self.status = "Test Status"
  
  
  class MainPage(webapp2.RequestHandler):
       def get(self):
            a = test()
            a.put()           
  
  
  app = webapp2.WSGIApplication([('/', MainPage)], debug=True)
  </code></pre>
  
  <p>Error: </p>
  
  <pre><code>  File "/home/user/Appengine/guestbook/guestbook.py", line 15, in get
      a = test()
    File "/home/user/Appengine/guestbook/guestbook.py", line 10, in __init__
      self.status = "Test Status"
    File "/home/user/google_appengine/google/appengine/ext/ndb/model.py", line 1265, in __set__
      self._set_value(entity, value)
    File "/home/user/google_appengine/google/appengine/ext/ndb/model.py", line 1011, in _set_value
      self._store_value(entity, value)
    File "/home/user/google_appengine/google/appengine/ext/ndb/model.py", line 992, in _store_value
      entity._values[self._name] = value
  TypeError: 'NoneType' object does not support item assignment
  </code></pre>