Decode ROT13'ed block of text

roturn.py

#ROTurn
#Decode a ROT13'ed block of text.
#Python version of Perl script by Jay Kominek
#By Robin Wellner (gvx)
#I hereby waive copyright and related or neighboring rights to this work
#See the Creative Commons Zero Waiver at <http://creativecommons.org/publicdomain/zero/1.0/>

"""
"I recently wrote a Perl script, which, using letter frequency, attempts
 to guess whether a given block of text has been ROT13ed or not. I've found
 that it works rather well (in my limited and very unscientific testing).
 The basis is that in plaintext, the letters i, s, e and t are very frequent,
 while their ROT13ed counterparts, v, f, r and g are relatively infrequent.
 However, in ROT13ed text, this is reversed. Hence by comparing which set
 of letters is predominant, you can make an awfully good guess as to which
 type of text you are dealing with."
    -- Jay Kominek (http://www.miranda.org/~jkominek/rot13/)
"""

def rotn(intext, n):
    outtext = []
    for char in intext:
        if 65 <= ord(char) < (65+26):
            char = chr(((ord(char) - 65) +n) % 26 + 65)
        elif 97 <= ord(char) < (97+26):
            char = chr(((ord(char) - 97) +n) % 26 + 97)
        outtext.append(char)
    return ''.join(outtext)

def rot13(intext):
    return rotn(intext, 13)

def rot_predict(intext):
    """Returns True if probably ROT13'ed, else False."""
    score = ( intext.count('i')
             +intext.count('s')
             +intext.count('e')
             +intext.count('t')
             -intext.count('v')
             -intext.count('f')
             -intext.count('r')
             -intext.count('g'))
    return score < 0

def roturn(intext):
    """Fixes ROT13'ed text, returns normal text untouched."""
    if rot_predict(intext):
        return rot13(intext)
    return intext