If you only want a return after each full stop, and not other sentence
ending punctuation, perhaps this would do the trick:
>>> text='''
AAAA
AA
A.
BBBBBB
B.
C.
DDDD
DD.
'''
... ... ... ... ... ... ... ... ... ... ... ... >>>
>>> import re
>>> text=re.sub('\n','',text)
>>> text=re.sub('\.','.\n',text)
>>> print text
AAAAAAA.
BBBBBBB.
C.
DDDDDD.
Alex.