class_docstr: Class to index 'Not Null' condition forsingle field in parquet dataset
class_docstr_tok: ["Class","to","index","'Not","Null","'","condition","forsingle","field","in","parquet","dataset"]
code: def build_index(self, decoded_rows, piece_index):
      field_column = [row[self._column_name] for row in decoded_rows]
      if not field_column:
          raise ValueError(
              "Cannot build index for empty rows, column '{}'".format(
                  self._column_name
              )
          )
  
      for field_val in field_column:
          if field_val is not None:
              self._index_data.add(piece_index)
              break
  
      return self._index_data
  
imports: import numpy
  from collections import defaultdict
  from petastorm.etl import RowGroupIndexerBase
label_desc: Use image node_id 7 for calling the FieldNotNullIndexer obj's underlying member method code with example usage: obj.build_index(decoded_rows, piece_index) and returns: self