A function to convert a list of integers into a string of ranges of consecutive values.

list_to_ranges.py

def list_to_ranges(in_list):
    ''' Convert a list of integers into a string of ranges of consecutive values. '''
    l = in_list[:]
    l.sort()
    range_str = str(l[0])
    for i in xrange(1,len(l)-1): # Handle first & last separately
        if l[i] == l[i-1]+1 and l[i] == l[i+1]-1:
            pass
        elif l[i] != l[i-1]+1:
            range_str += ', ' + str(l[i])
        else:
            range_str += '-' + str(l[i])
    if len(l) > 1:
        if l[-1] == l[-2]+1:
            range_str += '-' + str(l[-1])
        else:
            range_str += ', ' + str(l[-1])
    del l
    return range_str

# Example call:
list_to_ranges([1, 2, 3, 4, 5, 7, 8, 9, 21, 22, 23]) #=> '1-5, 7-9, 21-23'