1 s = "1, 3-7, 12, 15, 18-21"

     2 answer = []

     3 single = "answer.append( %s )"

     4 many = "answer.extend(range(%s))"

     5 COMMA = ','

     6 MINUS = '-'

     7 for p in s.split(COMMA):

     8     if MINUS not in p:

     9         eval( single % (p) )

    10     else:

    11         p = p.replace( MINUS, COMMA) + '+ 1 '

    12         eval( many % (p) )

    13 print answer