Surround the thing you're appending in brackets: I think you're doing .append(x for x...), but it should be .append([x for x...]). The first is generator expression, and the second one is a list.

Well, I don't see where you declared new_names, so it's not surprising. But ok, let's say you did. Then, new_names = new_names.append(...) - I really don't know how this should work, and it probably doesn't. My guess is that .append() returns None, so you re-assigned new_names to None.

You need to return the result, not print it.