Mini Project_1: Running Key Cipher

Write a program that will accept a plaintext and running key. The program should print out the ciphertext associated to the inputs. To make sure that the encryption worked, the program should also print the original plaintext based on performing the decryption of the ciphertext. In addition, the program should continue to perform the encryption (and decryption) as long as the user wants to encrypt another plaintext.

"""
Checks if an input only has letters.
@param check_string The string to be checked.
@return True if the message doesn't contain only letters. 
"""   
def inputchecker(check_string):
    for x in check_string:
        if (96 < ord(x) < 123) == False:
            return True

"""
Encodes the text by the key
@param user_string The string to be encoded.
@param key Decides how much it will be moved.
@return True if the message doesn't contain only letters. 
"""
def encoder(text, key):
    text_list = list(map(str, text))
    key_list = list(map(str, key))

    # new list made by assigning a new ascii value
    new_text_list = [ord(x) - 97 for x in text_list]
    new_key_list = [ord(y) - 97 for y in key_list]

    # calculates the ascii of letters and generates the string
    formula(new_text_list, new_key_list)
    

"""
Provides a formula for encoded text
@param text The string to be encoded.
@param key Decides how much it will be moved.
@return None 
"""     
def formula(text, key):
    encoded_list = []
    final_list = []
    for i in range(len(text)):
        
        # adding an element to the list
        encoded_list.append((text[i] + key[i]) % 26)        

    for i in encoded_list:
        final_list.append(chr(i + 97))

    # in order to convert a list to string, use .join!!  
    final_str = ''.join(final_list)
    print()
    print('The encoded text: ' + final_str)
    

"""
Code inside this function will run when the program starts.
"""
def main():
    
    user_choice = 'y'       # default value

    while user_choice != 'n':
        print('Enter text (letters only): ')
        user_text = input().lower()     # making it a lower case
        while inputchecker(user_text) == True:
            print('Input must contain only letters. Try again: ')
            user_text = input().lower()
        
        print('Enter key (letters only): ')
        user_key = input().lower()      # making it a lower case
        while inputchecker(user_key) == True:
            print('Input must contain only letters. Try again: ')
            user_key = input().lower()

        encoder(user_text, user_key)
        print('The decoded text: ' + user_text)

        print()
        print('Run again? ')
        user_choice = input()

    
if __name__ == '__main__':
    main()

Sample Run: