n_lines += (n_line)

Remember that doing the above would add each element in the string to the new list, giving this as the result:

['1', ':', ' ', 'a', '2', ':', ' ', 'b', '3', ':', ' ', 'c']

which is not desired, obviously.

You could have also used the range function to make it a bit easier, check the solutions of other people.
Learn how to ennumerate.