Added some solutions to exercises written during REC Chennai workshop.
s = "1, 3-7, 12, 15, 18-21"
answer = []
single = "answer.append( %s )"
many = "answer.extend(range(%s))"
COMMA = ','
MINUS = '-'
for p in s.split(COMMA):
if MINUS not in p:
eval( single % (p) )
else:
p = p.replace( MINUS, COMMA) + '+ 1 '
eval( many % (p) )
print answer