Well the assertion messages of Python test framework are suboptimal.

the issue here is probably not about whether their solution is correct, but that the test output itself can (if misreading, but reasonably so) seem to contradict the instructions (as it may seem to claim that 'TTTTUUUUU' was the input)

..which I agree with. this is poor wording which assumes a different context for the solver than the solver is actually in

Your algorithm is not correct. You append an 'U' for every 'T' (not exactly, actually it's even weirder but you get the point).

Not a kata issue.

"'TTTTUUUUU' should equal 'UUUU'
'GCATUU' should equal 'GCAU'"

Say what now? I thought this was supposed to be just about converting Ts to Us. What is this?