Best practices in software engineering

import string

_lower_cipher = string.ascii_lowercase[13:] + string.ascii_lowercase[:13]
_upper_cipher = string.ascii_uppercase[13:] + string.ascii_uppercase[:13]

def encode(message):
    """
    Encode a message from English to ROT13
    
    Args:
        message (str): the English message to encode
    
    Returns:
        str: The encoded message
    
    Examples:
        >>> encode("Secretmessage")
        'Frpergzrffntr'
    """
    output = []
    for letter in message:
        if letter in string.ascii_lowercase:
            i = string.ascii_lowercase.find(letter)
            output.append(_lower_cipher[i])
        elif letter in string.ascii_uppercase:
            i = string.ascii_uppercase.find(letter)
            output.append(_upper_cipher[i])
    
    return "".join(output)


def decode(message):
    """
    Encode a message from ROT13 to English
    
    Args:
        message (str): the ROT13 message to encode
    
    Returns:
        str: The decoded message
    
    Examples:
        >>> encode("Frpergzrffntr")
        'Secretmessage'
    """
    output = []
    for letter in message:
        if letter in _lower_cipher:
            i = _lower_cipher.find(letter)
            output.append(string.ascii_uppercase[i])
        elif letter in _upper_cipher:
            i = _upper_cipher.find(letter)
            output.append(string.ascii_uppercase[i])
    
    return "".join(output)