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 indexed_values(self):
      return ["Field is Not Null"]
  
imports: import numpy
  from collections import defaultdict
  from petastorm.etl import RowGroupIndexerBase
label_desc: Use image node_id 5 for calling the FieldNotNullIndexer obj's underlying member method code with example usage: obj.indexed_values() and returns: list