code: def _random_token(self, bits=128):
          
          alphabet = string.ascii_letters + string.digits + '-_'
          # alphabet length is 64, so each letter provides lg(64) = 6 bits
          num_letters = int(math.ceil(bits / 6.0))
          return ''.join(random.choice(alphabet) for i in range(num_letters))
docs: Generates a random token, using the url-safe base64 alphabet.
          The "bits" argument specifies the bits of randomness to use.