python: return index of last character of a substring

find() and index() return index of first character of the input pattern within the searched string.
This returns the index of the ending character of the input pattern.

# Return index of last character of small_string
def end_index(big_string,small_string):
    assert small_string in big_string
    start_index=big_string.index(small_string)
    return start_index + len(small_string)