class_docstr: Abstract base class for protocol messages.
  
  Protocol message classes are almost always generated by the protocol
  compiler.  These generated types subclass Message and implement the methods
  shown below.
docstr: Returns the name of the field that is set inside a oneof group.
  
  If no field is set, returns None.
  
  Args:
    oneof_group (str): the name of the oneof group to check.
  
  Returns:
    str or None: The name of the group that is set, or None.
  
  Raises:
    ValueError: no group with the given name exists
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code: def WhichOneof(self, oneof_group):
      """Returns the name of the field that is set inside a oneof group.
  
      If no field is set, returns None.
  
      Args:
        oneof_group (str): the name of the oneof group to check.
  
      Returns:
        str or None: The name of the group that is set, or None.
  
      Raises:
        ValueError: no group with the given name exists
      """
      raise NotImplementedError
  
label_desc: Use image node_id 19 for calling the Message obj's underlying member method code with example usage: obj.WhichOneof(oneof_group) without return types