def add_item ( self, offset ):
        list, index = self.get_info()
        index      += offset
        item_trait  = self._trait_handler.item_trait
        value       = item_trait.default_value_for( self.object, self.name )
        self.value = list[:index] + [ value ] + list[index:]
        self.update_editor()

    #---------------------------------------------------------------------------
    #  Inserts a new item before the current item:
    #---------------------------------------------------------------------------
Adds a new value at the specified list index.
dataset/ETHPy150Open enthought/traitsui/traitsui/qt4/list_editor.py/SimpleEditor.add_item