python - Insert values in lists following a pattern -

i have following example list:

list_a = [(1, 6),            (6, 66),            (66, 72),            (72, 78),            (78, 138),            (138, 146),            (154, 208),            (208, 217),            (217, 225),            (225, 279),            (279, 288)           .....           ] 

and need is:

1. after every 6 elements on list, insert in place new tuple formed last number of previous one, , first number in previous 6 tuples.
2. after tuple inserted, insert formed first number of previous 1 plus 1, , last number of previous 1 , first number of next tuple.

so result may like:

list_a = [(1, 6),            (6, 66),            (66, 72),            (72, 78),            (78, 138),            (138, 146),           (146, 1),     # <- first part            (147, 154),   # <- second part           (154, 208),            (208, 217),            (217, 225),            (225, 279),            (279, 288)           (288, 147)     # <- first part           .....           ] 

i have tried this, last last elements missing

for in range(0, len(list_a)+1, 6):     if > 0:         list_a.insert(i, (list_a[i - 1][1], list_a[i - 6][0]))         list_a.insert(i + 1, (list_a[i - 1][1] + 1, list_a[i + 1][0],)) 

i build new list appending rather inserting existing list. should work:

n = len(list_a) newlist = [] in range(0,n, 6):     newlist.append(list_a[i:i+6] )       newtuple1 = (newlist[-1][1], newlist[i][0])     newlist.append(newtuple1)     try:         newtuple2 = (newtuple1[0] + 1, list_a[i+6][0])         newlist.append(newtuple2)     except indexerror:         print "there no next tuple"  print newlist 

output

there no next tuple [(1, 6), (6, 66), (66, 72), (72, 78), (78, 138), (138, 146), (146, 1), (147, 154), (154, 208), (208, 217), (217, 225), (225, 279), (279, 288), (300, 400), (400, 146)] 

note example did not indicate in case 2 if there no additional tuples. supposed there 12 tuples in list_a. when second group of 2, there no next tuple.

hope helps.