code: def value_to_python(self, value):
          
          if not isinstance(value, bytes):
              raise tldap.exceptions.ValidationError("should be a bytes")
  
          length = len(value) - 8
          if length % 4 != 0:
              raise tldap.exceptions.ValidationError("Invalid sid")
  
          length = length // 4
  
          array = struct.unpack('<bbbbbbbb' + 'I' * length, value)
  
          if array[1] != length:
              raise tldap.exceptions.ValidationError("Invalid sid")
  
          if array[2:7] != (0, 0, 0, 0, 0):
              raise tldap.exceptions.ValidationError("Invalid sid")
  
          array = ("S", ) + array[0:1] + array[7:]
          return "-".join([str(i) for i in array])
docs: Converts the input single value into the expected Python data type,
          raising django.core.exceptions.ValidationError if the data can't be
          converted.  Returns the converted value. Subclasses should override
          this.