ROT-13

gistfile1.py

>>> from string import ascii_lowercase, ascii_uppercase, maketrans, translate
>>> target = ascii_lowercase[13:] + ascii_lowercase[:13] + ascii_uppercase[13:] + ascii_uppercase[:13]
>>> tr = maketrans(ascii_lowercase + ascii_uppercase, target)
>>> translate("This is a message", tr)
'Guvf vf n zrffntr'

>>> "This is a message.".encode('rot13')
'Guvf vf n zrffntr.'