No I tested different numbers, because I haven't noticed test numbers... I tested 5, 27, 9, 25, 81, and few similar examples and they all worked correct and now actually tried with -1 and there is a bug, thanks.

At the end I can say that there was only one exception for -1 and the rest works correct... Thanks for your answer, I finally solved!

But what numbers do you test in your local Python interpreter? Do you test the same numbers as Codewars does? Did you test is_square(-1) (which you can see in the error message)?

Sorry about that issue mark, I'm first time here.

As I understood problem is that here interpreter shows kind of mistake which never happens at least with my local Python interpreter.

Anyway, it is a question, not an issue (yet).

Are you testing it there with that input value?