I agree that None as an element presents an issue for this solution, but where in the specification is a sequence itself prevented from being an element in the sequence?

For example:

>>>> unique_in_order(['abc','abc']) 
['abc']

Nice, but this will not work in case text contains uppercase characters.

Just see pydoc itertools.groupby.

I think the better way would be to assign '' to prev. Since '' is itself a sequence, it should not occur as per the question's specifications.