def is not the only way to define a function in Python.

how do i dont use 'def' and 'return' when codwars want to call my code with def?

The ISSUE tag is reserved for critical flaws with the tests. 718 people have solved this Kata in JS before just fine, so, when in doubt, please use the QUESTION tag instead. As for your question, the code inside your loop needs correction. Instead of focusing on the complicated test that fails, focus on the other more simple test that the solution fails on: f: abcd s: dabcd returns abcd for you when it should return abcdabcd. This is a byproduct of the method you use to calculate indexInSecond.

Yes, the issue resutls from your code misunderstanding the task. The only overlap that this Kata is interested in is between the end of the first string and the start of the other. A string overlapping in the middle of the other doesn't count. As you've noticed, merge_strings('xabc', 'ab') is expected to return xabcab (since no overlap on the edges), and not xabc. Consult the Kata description further if in doubt

[1  2  (3  4]  5)

That's what it means they overlap and become a single interval from 1 to 5.

Anyway, why does it says "Since [1, 4] and [3, 5] is overlap"?

Hi, I'm a bit confused about the example on overlapping interval. I thought the sum would be 8 but it said 7 instead. Am I missing something?

Thanks it worked

Please see this paragraph: https://docs.codewars.com/training/training-example#debugging-a-kata for some hints how to debug your solution.

No
How can i see it

Please see this paragraph: https://docs.codewars.com/training/training-example#debugging-a-kata for some hints how to debug your solution.

Do you know what inputs make you fail?