ROT-n cipher in Python 3, defaulting as ROT-13

rot13.py

from string import ascii_lowercase, ascii_uppercase

# Note that this already exists in Python as codecs.encode("text", "rot13")
# so you'd probably wanna use that instead of this implementation,
# unless you need a rotation of n places with n != 13.


def rot(text, rotation=13):
    """ROT-n cipher.
    Decode by running the output through the function again
    (assuming the rotation is exactly half of the used character set)."""
    result = ""
    for char in text:
        if char.islower():
            charset = ascii_lowercase
        else:
            charset = ascii_uppercase
        pos = charset.find(char)
        if pos == -1:
            result += char  # Not in charset? Append as-is.
            continue
        pos = (pos + rotation) % len(charset)
        result += charset[pos]
    return result


assert rot("foobar") == "sbbone"
assert rot("sbbone") == "foobar"
assert rot(rot("foobar")) == "foobar"
assert rot("Abc!") == "Nop!"
assert rot("Nop!") == "Abc!"
assert rot(rot("Abc!")) == "Abc!"