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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 _column_names(self):
 """Return the column names"""

        index_names = set(_normalize_index_names(self._index_names))
        column_names = [col_name for col_name in self._schema_rdd.columns if
                        col_name not in index_names]
        return column_names