def singleline_diff(line1, line2):
    """
    Inputs:
      line1 - first single line string
      line2 - second single line string
    Output:
      Returns the index where the first difference between
      line1 and line2 occurs.

      Returns IDENTICAL if the two lines are the same.
    """
    diffidx = -1
    # minlength establishes loop for identical case and
    # cases where differences found before string end
    minlength = min(len(line1), len(line2))
    for idx in range(minlength):
        if line1[idx] != line2[idx]:
            diffidx = idx
            break

    # if diffidx is still None confirm strings are not of unequal length
    if diffidx == -1 and len(line1) != len(line2):
        diffidx = minlength

    return diffidx