(Codewars) Range Extraction Kata: https://www.codewars.com/kata/range-extraction/python


  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. It is not considered a range unless it spans at least 3 numbers. For example ("12, 13, 15-17")  
  
  
  Complete the solution so that it takes a list of integers in increasing order and returns a correctly formatted string in the range format.
  
  Example:

solution([-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20])
 # returns "-6,-3-1,3-5,7-11,14,15,17-20"

  
  Courtesy of rosettacode.org  


My Code

def solution(lst):
    res = []
    if lst:
        tmp, i, ln = lst[0], 0, len(lst)
        while i &lt; ln:
            tmp, j = lst[i], i
            while j &lt; ln - 1 and lst[j+1] == lst[j]+1:
                j += 1
            if j - i &gt; 1:
                tmp = str(lst[i]) + "-" + str(lst[j])
                i = j+1
            else:
                i = (j if j &gt; i else i+1)
            res.append(tmp)
    return ",".join(str(x) for x in res)
Grouping

If you need to group anything using; groupby is probably the easiest way

Here we can use a itertools.groupby recipe to group the consecutive ranges:

&gt;&gt;&gt; for _, g in groupby(enumerate(lst), lambda i_x : i_x[0] - i_x[1]):
...     print([x for _, x in g])
[-6]
[-3, -2, -1, 0, 1]
[3, 4, 5]
[7, 8, 9, 10, 11]
[14, 15]
[17, 18, 19, 20]


Yielding the correct representation

Now all is left is to check whether there are more then 2 items in the range, and return the range. Else return all the numbers in the range normally

We can make this a generator, which yields either the range or a number, and afterwards join them together.

Full Code

def group_consecutives(lst):
    for _, g in groupby(enumerate(lst), lambda i_x : i_x[0] - i_x[1]):
        r = [x for _, x in g]
        if len(r) &gt; 2:
            yield f"{r[0]}-{r[-1]}"
        else:
            yield from map(str, r)

def range_extraction(lst):
    return ','.join(group_consecutives(lst))