A format for expressing an ordered list of integers is to use a comma separated list of either

 individual integers
 Or a range of integers denoted by the starting integer separated from the end integer in the range by a dash, '-'. (The range includes all integers in the interval including both endpoints)
 The range syntax is to be used only for, and for every range that expands to more than two values.
Example

The list of integers:

 -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20
Is accurately expressed by the range expression:

 -6,-3-1,3-5,7-11,14,15,17-20
(And vice-versa).

Task

 Create a function that takes a list of integers in increasing order and returns a correctly formatted string in the range format. 
 Use the function to compute and print the range formatted version of the following ordered list of integers. (The correct answer is: 0-2,4,6-8,11,12,14-25,27-33,35-39).


    0,  1,  2,  4,  6,  7,  8, 11, 12, 14,
   15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
   25, 27, 28, 29, 30, 31, 32, 33, 35, 36,
   37, 38, 39

 Show the output of your program.


Related task

   Range expansion


def range_extract(lst):
    'Yield 2-tuple ranges or 1-tuple single elements from list of increasing ints'
    lenlst = len(lst)
    i = 0
    while i< lenlst:
        low = lst[i]
        while i <lenlst-1 and lst[i]+1 == lst[i+1]: i +=1
        hi = lst[i]
        if   hi - low >= 2:
            yield (low, hi)
        elif hi - low == 1:
            yield (low,)
            yield (hi,)
        else:
            yield (low,)
        i += 1
 
def printr(ranges):
    print( ','.join( (('%i-%i' % r) if len(r) == 2 else '%i' % r)
                     for r in ranges ) )
 
if __name__ == '__main__':
    for lst in [[-8, -7, -6, -3, -2, -1, 0, 1, 3, 4, 5, 7,
                 8, 9, 10, 11, 14, 15, 17, 18, 19, 20],
                [0, 1, 2, 4, 6, 7, 8, 11, 12, 14, 15, 16, 17, 18, 19, 20, 21, 22,
                 23, 24, 25, 27, 28, 29, 30, 31, 32, 33, 35, 36, 37, 38, 39]]:
        #print(list(range_extract(lst)))
        printr(range_extract(lst))