Yes,correcting myself. I should aim indexes which actual idlelib uses. Thank you Terry for the grammar. Fighting with the index parsing last week, and finally came up with a level to break the index into base and modifier and decode it along with using regular expressions.
Now the mock_tk attached at this patch passes all the existing test cases, and failing the new test I have written on text_test

The problem now I am dealing with is, there are four exceeding conditions I have to handle which make errors.
1.returning line  < 0
2.returning line  > last line
3.returning char  < 0 
4.returning char > its line length

For example, in parsing base the return line may exceed last line but the modifier will say '-2lines'  doing that, now the return line is not exceeding last line.