Returns the format fields otherwise raise exception

def parse_fields(cls, formatter):
        "Returns the format fields otherwise raise exception"
        if formatter is None: return []
        try:
            parse = list(string.Formatter().parse(formatter))
            return  set(f for f in list(zip(*parse))[1] if f is not None)
        except:
            raise SyntaxError("Could not parse formatter %r" % formatter)