Question

I want to be able to take a dynamically created string, say “Pigeon” and determine at runtime whether Google App Engine has a Model class defined in this project named “Pigeon”. If “Pigeon” is the name of a existant model class, I would like to then get a reference to the Pigeon class so defined.

Also, I don’t want to use eval at all, since the dynamic string “Pigeon” in this case, comes from outside.

Answer 1

There’s two fairly easy ways to do this without relying on internal details:

Use the google.appengine.api.datastore API, like so:

from google.appengine.api import datastore

q = datastore.Query('EntityType')
if q.get(1):
  print "EntityType exists!"

The other option is to use the db.Expando class:

def GetEntityClass(entity_type):
  class Entity(db.Expando):
    @classmethod
    def kind(cls):
      return entity_type
  return Entity

cls = GetEntityClass('EntityType')
if cls.all().get():
  print "EntityType exists!"

The latter has the advantage that you can use GetEntityClass to generate an Expando class for any entity type, and interact with it the same way you would a normal class.