So We compare each integer representation against lowercase and uppercase letters. We subtract or add 13 to shift characters.

And We append to the result string the character representation of the integer. We call chr() to do this.

def rot13(s):
result = ""# Loop over characters.for v in s:
# Convert to number with ord.
c = ord(v)
# Shift number back or forward.
if c >= ord('a') and c <= ord('z'):
if c > ord('m'):
c -= 13
else:
c += 13
elif c >= ord('A') and c <= ord('Z'):
if c > ord('M'):
c -= 13
else:
c += 13
# Append to result.
result += chr(c)
# Return transformation.
return result
# Test method.
print(rot13("gandalf"))
print(rot13(rot13("gandalf")))tnaqnys
gandalf

Notes, tests. We test to make sure the rot13() method is correct. When we call rot13() on the result of rot13(), we again have our original string value.

Thus The rot13() algorithm round-trips its data correctly—the original data is not lost. This is correct.

Discussion. The ROT13 algorithm is an important one to implement. It is a handy way to add obscurity—text that is not easily readable. This is useful in certain situations.

Also The algorithm helps teach us how to treat characters as numbers. In Python we do this with ord() and chr(), two built-in functions.

Translate A faster implementation is possible using the translate method. We benchmark and implement one.