Vigenere Cypther

vigenere.py

import base64


def encode(key, string)
    encoded_chars = []
    for i in xrange(len(string)):
        key_c = key[i % len(key)]
        encoded_c = chr(ord(string[i]) + ord(key_c) % 256)
        encoded_chars.append(encoded_c)
    encoded_string = "".join(encoded_chars)
    return base64.urlsafe_b64encode(encoded_string)


def decode(key, string):
    decoded_chars = []
    string = base64.urlsafe_b64decode(string)
    for i in xrange(len(string)):
        key_c = key[i % len(key)]
        encoded_c = chr(abs(ord(string[i]) - ord(key_c) % 256))
        decoded_chars.append(encoded_c)
    decoded_string = "".join(decoded_chars)
    return decoded_string

Missing the colon on line 4.

You're missing a colon on line 4. def encode(key, string) should be def encode(key, string):.

Modulo operator (%) has higher precedence than either + or -, so line 8 is missing parens:

encoded_c = chr( (ord(string[i]) + ord(key_c)) % 256)

Ditto line 19.