old_contents: class Masker:
      """
      Data sent from the server must be masked to prevent malicious clients
      from sending data over the wire in predictable patterns.
  
      Servers do not have to mask data they send to the client.
      https://tools.ietf.org/html/rfc6455#section-5.3
      """
  
      def __init__(self, key):
          self.key = key
          self.offset = 0
  
      def mask(self, offset, data):
          result = bytearray(data)
          for i in range(len(data)):
              result[i] ^= self.key[offset % 4]
              offset += 1
          result = bytes(result)
          return result
  
      def __call__(self, data):
          ret = self.mask(self.offset, data)
          self.offset += len(ret)
          return ret
  
new_contents: import sys
  
  
  class Masker:
      """
      Data sent from the server must be masked to prevent malicious clients
      from sending data over the wire in predictable patterns.
  
      Servers do not have to mask data they send to the client.
      https://tools.ietf.org/html/rfc6455#section-5.3
      """
  
      def __init__(self, key):
          self.key = key
          self.offset = 0
  
      def mask(self, offset, data):
          datalen = len(data)
          offset_mod = offset % 4
          data = int.from_bytes(data, sys.byteorder)
          num_keys = (datalen + offset_mod + 3) // 4
          mask = int.from_bytes((self.key * num_keys)[offset_mod:datalen +
                                                      offset_mod], sys.byteorder)
          return (data ^ mask).to_bytes(datalen, sys.byteorder)
  
      def __call__(self, data):
          ret = self.mask(self.offset, data)
          self.offset += len(ret)
          return ret
  
subject: replace masker with more optimized one