I like the katana, but i missed some information for the "rules of scoring". I'm not familiar with tennis sport itself, but thanks to the katana i searched for some more information about counting in tennis. Maybe it would be nice to enhance the rules with:
A fourth point win by a player decides the game for him if the player then has a lead of at least two points, i.e. after the scores 40:0, 40:15 and 40:30 (or vice versa). If the score is 40:40, it is called a deuce. The following point is called an advantage.

Thanks for your help. I updated my code and now it is working. Now i also understand the test itself. You used the same list for a couple of tests, that's why my modification had this strong influence to the further tests. I'm sorry. I will keep this in mind for furhter katas.

Would you recommend to work always on a copy of the inputs?

Hi g964, i did what you said and printed the first element of ls with print(ls[0]).

I got this:
Traceback (most recent call last):
File "main.py", line 26, in
Test.assert_equals(choose_best_sum(880, 8, xs), 876)
File "/home/codewarrior/solution.py", line 2, in choose_best_sum
print(ls[0])
IndexError: list index out of range

Can you please check the tests, there must be something wrong.

greetings EScoder