code: def custom_serialized(cls, serialized, is_java=True):
      
      if not isinstance(serialized, bytes):
        raise TypeError("Argument to custom_serialized() must be "
                        "a serialized Python class as bytes, given: %s" % str(serialized))
      if not is_java:
        return cls.CUSTOM(gtype=topology_pb2.Grouping.Value("CUSTOM"),
                          python_serialized=serialized)
      else:
        raise NotImplementedError("Custom grouping implemented in Java for Python topology"
                                  "is not yet supported.")
docs: Custom grouping from a given serialized string
  
      This class is created for compatibility with ``custom_serialized(cls, java_serialized)`` method
      of StreamParse API, although its functionality is not yet implemented (Java-serialized).
      Currently only custom grouping implemented in Python is supported, and ``custom()`` method
      should be used to indicate its classpath, rather than directly to use this method.
  
      In the future, users can directly specify Java-serialized object with ``is_java=True`` in order
      to use a custom grouping implemented in Java for python topology.
  
      :param serialized: serialized classpath to custom grouping class to use (if python)
      :param is_java: indicate whether this is Java serialized, or python serialized