pretty dumb question but I don't know

edit

en_alphabet = "abcdefghijklmnopqrstuvwxyz"
def is_alphabetic_char(c):
    return (c.lower() in en_alphabet)
def char_to_num(c):
    return en_alphabet.index(c.lower())
def num_to_char(x):
    return en_alphabet[x % 26]

def CaesarEncrypt(k, plaintext):
    ciphertext=""
    for j in xrange(len(plaintext)):
        p= plaintext[j]
        if is_alphabetic_char(p):
            x= (k + char_to_num(p))%26
            c= num_to_char(x)
        else:
            c = p
            ciphertext += c
        return ciphertext

(but when I enter

k=16 ;plaintext = 'test.'
CaesarEncrypt(k,plaintext)

)

why does it reply me ' ' but ciphertext......