collapse a list of chars into a range

collapse.py

def collapse(lst):
  def chunk(lst):
    ret = [lst[0],]
    for i in lst[1:]:
      if ord(i) == ord(ret[-1]) + 1:
        pass
      else:
        yield ret
        ret = []
      ret.append(i)
    yield ret
  chunked = chunk(lst)
  ranges = ((min(l), max(l)) for l in chunked)
  return ",".join("{0}-{1}".format(*l) if l[0] != l[1] else l[0] for l in ranges)


print collapse(['a', 'b', 'c', 'd', 'f', 'g', 'h', 'q'])
print collapse(['a', 'b', 'c', 'd', 'f', 'g', 'h'])
print collapse(['a', 'c', 'f', 'h'])