ROT-N variable-offset substitution encipher for Python3 (to decipher, enter negative offset)

rot_n.py

from string import ascii_lowercase

def rot_n(in_string, n=13):
    "returns copy of in_string shifted alphabetically by given offset N"
    n %= len(ascii_lowercase)
    char_shifts = ascii_lowercase[n:] + ascii_lowercase[:n]
    out_string = ''
    for char in in_string:
        if not char.isalpha():
            out_string += char
            continue
        new_char = char_shifts[ascii_lowercase.find(char.lower())]
        out_string += new_char.upper() if char.isupper() else new_char
    return out_string

if __name__ == '__main__':
    # for Python2 compat use raw_input instead
    in_string = input('String to shift: ')
    offset = int(input('Offset to shift string by (default 13): ') or 13)
    print(rot_n(in_string, offset))