(a) This type of thing is usually frowned-upon in Python circles:
''.join([chr((ord(c) - 84) % 26 + 97) for c in s])
(b) This could be achieved with two list comprehensions as follows:
' '.join([''.join([chr((ord(c) - 84) % 26 + 97) for c in w]) for w in s.split()])