Caesar Cipher Help

[pbrowne]

The following Caesar Cypher code is not working (Python 3). The text file is being written, but with incorrect cipher.

e.g. Input text as 'Hello' should be written as 'Uryyb!' with a key of 13. Instead it is being written as 'b'

# Caesar cypher function
def rot(text, key):

    # Iterate through each character in the message.
    for char in text:
        # Set cypher_text to an empty string to add to later
        cypher_text = ''
        # Check if the character is a letter (A-Z/a-z).
        if char.isalpha():
            # Get the Unicode number from of the character.
            num = ord(char)
            # If the final number is greater than 'z'/122..
            if (num + key) > 122:
                # If we go too far, work out how many spaces passed
                # 'a'/97 it should be using the proper key.
                x = (num + key) - 122
                # Add the chr version of x more characters passed
                # 'a'/97, take 1 to account for the 'a' position.
                # This adds a string character on to the cypher_text
                # variable.
                cypher_text += chr(x + ord('a') - 1)
            # If the rotated value doesn't go passed 'z'/122
            elif ((num + key <= 122)):
                # Use the key to add to the decimal version of the
                # character, add the chr version of the value to the
                # cypher text.
                cypher_text += chr(num + key)
        # Else, if the chracter is not a letter, simply add it as is.
        # This way we don't change symbols or spaces.
        else:
            cypher_text += char
    # Return the final result of the processed characters for use.
    return cypher_text


# Ask the user for their message
plain_input = input('Input the text you want to encode: ')
# Aks the user for the rotation key
rot_key = int(input('Input the key you want to use from 0 to 25: '))
# Secret message is the result of the rot function
secret_message = rot(plain_input, rot_key)
# Print out message for feedback
print('Writing the following cypher text to file:', secret_message)
# Write the message to file
with open('TestFile.txt', 'a+') as file:
    file.write(secret_message)