Taken task based approach for Session 3, day 1.
Removed introduction to dictionaries slide and accommodated the same
content while solving the problem.
str_ranges = "1, 3-7, 12, 15, 18-21"
ranges = str_ranges.split(',')
lst = []
for r in ranges:
vals = r.split('-')
if len(vals) == 2:
lst.extend(range(int(vals[0]), int(vals[1]) + 1))
else:
lst.append(int(vals[0]))
print lst