AppEngine DB Property validate

def validate(self, value):
    """Assert that provided value is compatible with this property.

    Args:
      value: Value to validate against this Property.

    Returns:
      A valid value, either the input unchanged or adapted to the
      required type.

    Raises:
      BadValueError if the value is not appropriate for this
      property in any way.
    """
    if self.empty(value):
      if self.required:
        raise BadValueError('Property %s is required' % self.name)
    else:
      if self.choices:
        if value not in self.choices:
          raise BadValueError('Property %s is %r; must be one of %r' %
                              (self.name, value, self.choices))
    if self.validator is not None:
      self.validator(value)
    return value