Ndb StringProperty validation

model.py

def _validate(self, value):
    if isinstance(value, str):
      # Decode from UTF-8 -- if this fails, we can't write it.
      try:
        value = unicode(value, 'utf-8')
      except UnicodeError:
        raise datastore_errors.BadValueError('Expected valid UTF-8, got %r' %
                                             (value,))
    elif not isinstance(value, unicode):
      raise datastore_errors.BadValueError('Expected string, got %r' %
                                           (value,))