'Return boolean indicating wheter parameter is optional.'

def parse_optional(self, default=None):

    elem = self.input_elem
    if (self.get('type') == 'data_column'):
        if ('force_select' in elem.attrib):
            force_select = string_as_bool(elem.get('force_select'))
        else:
            force_select = (not string_as_bool(elem.get('optional', False)))
        return (not force_select)
    if (default is None):
        default = self.default_optional
    return self.get_bool('optional', default)
