nub in your solution has pitiful perfomance.

Not an issue (at this stage).

This is a fair requirement, but should it be mentioned as part of the instructions, or be visible in the user-facing test suite? If there are specific requirements around exception handling, I think the user should be able to see them.

Okay, I've added a test to make sure that copy_list raises an exception when you hand it None.

Please close this issue.

I'd expect a solution to throw an exception if I gave it None as input... so I will add this test.