code: def _from_base_type(self, value):
          
          if not value:
              return None
          try:
              # Uses the from_json method of the implied class of value
              credentials = client.Credentials.new_from_json(value)
          except ValueError:
              credentials = None
          return credentials
docs: Converts our stored JSON string back to the desired type.
  
          Args:
              value: A value from the datastore to be converted to the
                     desired type.
  
          Returns:
              A deserialized Credentials (or subclass) object, else None if
              the value can't be parsed.