'Initialize shift layer.
Args:
vocab: (list of String) the vocabulary
shift: (Integer) the amount of shift apply to the alphabet.
Positive number implies shift to the right, negative number
implies shift to the left.'

def __init__(self, vocab, shift):

    self.shift = shift
    alphabet = vocab
    shifted_alphabet = deque(alphabet)
    shifted_alphabet.rotate(shift)
    self.encrypt = dict(zip(alphabet, list(shifted_alphabet)))
    self.decrypt = dict(zip(list(shifted_alphabet), alphabet))
