def identity(item, args):
    """Return the value from one column and an empty string.
       (ie, return a key and '' for no subkey).

       YAML usage:

       outputs:
         outcol:
           function: identity
           arguments: [KEYCOL]

       Where KEYCOL is the column name for the key to return.  The
       col() pseudo-function will convert the column name to a 0-based
       index offset to the row.
    """
    if len(item) <= args[0]:
        return '', ''
    return item[args[0]], ''
