OP solved it, closing. Also description has been updated

It is not considered a range unless it spans at least 3 numbers. For example "12,13,15-17"

So, your example above should return -54,-53 and -68--66 respectively.

Please use markdown tags to format your code or it's unusable, see there: https://docs.codewars.com/training/troubleshooting/#post-discourse

Negative values in this kata might look strange after formatting, but they don't really need any special handling. You process negative values exactly the same way as positive ones.
You can show your code so maybe we could find your problem and help you.

Please only mark issue for kata issues.

arr[len(arr)] is out of range since indexing starts at 0.

Also, you can use markdown to format your code.