@staticmethod
def get_description(v, item):
    """
        Return description of parameter.

        :param v: VOEvent xml
        :param item: single dictionary item from mapping
        :type v: lxml.objectify.ObjectifiedElement
        :type item: dict
        :returns: Description on parameter is applicable, else None
        :rtype: str, NoneType
        """
    param_group = item.get('param_group')
    param_name = item.get('param_name')
    try:
        note = v.find(".//Group[@name='{}']".format(param_group)).find(
            ".//Param[@name='{}']".format(param_name)).Description
        if note:
            return '[{}] {}'.format(param_name, note)
        else:
            return None
    except AttributeError:
        return None
