'join' is a method on an array object. You are not returning an array, so no .join method exists.

many random tests failing saying one fewer rider is needed than is actually needed by problem domain.

From a cs perspective, every time you change a data type new memory is allocated and time complexity is increased (Big-O). Python does memory management under the hood, so it is easy to ignore. In a lot of ways the brilliance of the Python interpreter is that is this black box that you throw code into and it does some magic and spit out what you asked for. This is a big topic, though, more than a comment forum. If you are working with small data in a low use environment, memory managment isn't a big deal. If you are paying the AWS bill, best practices become real important.

clever, yes, but best practice would not include changing data type (int -> str).