An Implementation of the caesar-cipher and rot13

cipher.py

def encipher(letter,shift):
    letter = letter.upper() # just to make sure the letter is uppercase
    ascii_letter = ord(letter)
    ascii_A = ord('A')
    pos_letter = ascii_letter - ascii_A
    shifted_pos_letter = ( pos_letter + shift )%26
    asc_ciphered_letter = shifted_pos_letter + ascii_A
    return chr(asc_ciphered_letter)

def encipher_text(text,shift):
    ciphertext = ''
    for c in text:
        if c.isalpha():
            ciphertext += encipher(c,shift)
        else :
            ciphertext += c
    return ciphertext
    
def rot13(text):
    return encipher_text(text,13)