def METHOD_NAME(self, value):
    """Validate the service keyword
    """
    current_value = self.context.getKeyword()
    # Check if the values changed
    if current_value == value:
        # nothing changed
        return
    # Check if the new value is empty
    if not value:
        return _("Keyword required")
    # Check the current value with the validator
    validator = ServiceKeywordValidator()
    check = validator(value, instance=self.context)
    if isinstance(check, string_types):
        return _(check)
    return None