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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_source_data_items(self, data_item: DataItem) -> typing.List[DataItem]: """Return the list of data items that are data sources for the data item. :return: The list of :py:class:`nion.swift.Facade.DataItem` objects. .. versionadded:: 1.0 Scriptable: Yes """

        return [DataItem(data_item) for data_item in self._document_model.get_source_data_items(data_item._data_item)] if data_item else None