python - Creating a Google App Engine datastore entity without initializing a particular property -


I have a class entity, which has the property name , and students . I want to make the unit first, then add value to student property later. But I'm not sure how to do this. I've tried null (I'm sorry if this is disgusting, new to Python) but it gives an error message. This is my class definition 

  class category (ndb.model): name = ndb.StringProperty (required = true) students = ndb.KeyProperty (repeated = true) ...   
 and handler for my class model  
  class classhandler (webapp2.RequestHandler): def post (self): name = self.request .get ('class_name') callback = self.request.get ('callback') class = class.get_by_id (name) if class: Self.error (409) Other: n = self.request.get ('class_name') Class = class (id = n, class_name = n, member) # & lt; & Lt; & Lt; --- // This is the place where the stuck team. Put () If Callback: self.response.write (call Previous + '(' + Team to JSN (+) ') Other: Write a self-response. (Team.Jason ())   
 I have been looking at a solution for the last two hours on Google. Any suggestions on what I have to do will be appreciated.  
  Edit:  I get this error message when I  null   Enter image details here    
 
 
 I'm not sure when you are talking about "null" or someone, then what are you talking about.  
 To add such students Any method that you need to provide list of students in the constructor       Html>

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