How to do an inverse `range`, i.e. create a compact range based on a set of numbers?

Question

Python has a range method, which allows for stuff like:

>>> range(1, 6)
[1, 2, 3, 4, 5]

What I’m looking for is kind of t

Answer 1

Sort numbers, find consecutive ranges (remember RLE compression?).

Something like this:

input = [5,7,9,8,6, 21,20, 3,2,1, 22,23, 50]

output = []
first = last = None # first and last number of current consecutive range
for item in sorted(input):
  if first is None:
    first = last = item # bootstrap
  elif item == last + 1: # consecutive
    last = item # extend the range
  else: # not consecutive
    output.append((first, last)) # pack up the range
    first = last = item
# the last range ended by iteration end
output.append((first, last))

print output

Result: [(1, 3), (5, 9), (20, 23), (50, 50)]. You figure out the rest :)