ranges.py

import itertools
import operator

def range_str(l):
    groups=[]
    for _, g in itertools.groupby(enumerate(l), lambda (i,x):i-x):
        group = map(operator.itemgetter(1), g)
        if len(group) > 1:
            groups.append("{}-{}".format(group[0], group[-1]))
        else:
            groups.append(str(group[0]))
    return ",".join(groups)

print(range_str([11,12,15,16,17,18]))

This is great! Thanks!