def fraction(item, args): # args = numerator_col, denominatior_col
    """Return the value stored in a column divided by the value in another

       YAML usage:

       outputs:
         outcol:
           function:         identity
           arguments:        [col(KEYCOL)]
           value:            fraction
           value_arguments:  [VALCOL_NUMERATOR, VALCOL_DENOMINATOR]
    """
    return float(item[args[0]]) / float(item[args[1]])