human
I wrote the below code

    if (not orderby):
        return
    db = current.db
    adapter = S3DAL()
    COMMA = adapter.COMMA
    INVERT = adapter.INVERT
    if isinstance(orderby, str):
        items = orderby.split(',')
    elif (type(orderby) is Expression):
        def expand(e):
            if isinstance(e, Field):
                return [e]
            if (e.op == COMMA):
                return (expand(e.first) + expand(e.second))
            elif (e.op == INVERT):
                return ([e] if expr else [e.first])
            return []
        items = expand(orderby)
    elif (not isinstance(orderby, (list, tuple))):
        items = [orderby]
    else:
        items = orderby
    s3db = current.s3db
    tablename = (table._tablename if table else None)
    for item in items:
        if (type(item) is Expression):
            if (not isinstance(item.first, Field)):
                continue
            f = (item if expr else item.first)
        elif isinstance(item, Field):
            f = item
        elif isinstance(item, str):
            (fn, direction) = (item.strip().split() + ['asc'])[:2]
            (tn, fn) = ([tablename] + fn.split('.', 1))[(-2):]
            if tn:
                try:
                    f = s3db.table(tn, db_only=True)[fn]
                except (AttributeError, KeyError):
                    continue
            elif current.response.s3.debug:
                raise SyntaxError(('Tablename    prefix    required    for    orderby="%s"' % item))
            else:
                continue
            if (expr and (direction[:3] == 'des')):
                f = (~ f)
        else:
            continue
        (yield f)

What's a good function header?

Write me a Python code to fix    required    for    orderby="%s"' % item))?
assistant
#Here is Python code to fix    required    for    orderby="%s"' % item))
.

def s3_orderby_fields(table, orderby, expr=False):