>>> string = "this is string"
>>> dict = {}
>>> y=0
>>> for x in string.strip():
...     if x in dict.keys():
...             dict[x] = [dict[x], y]
...     else:
...             dict[x] = y
...     y +=1
...
>>> dict
{'t': [0, 9], 'h': 1, 'i': [[2, 5], 11], 's': [[3, 6], 8], ' ': [4, 7], 'r': 10, 'n': 12, 'g': 13}