class_docstr: Utility class to resolve Placeholder expressions.
  
  Placeholder expression is defined as a proto structure
  placeholder_pb2.PlaceholderExpression. It can be resolved with
  ResolutionContext to a concrete value.
class_docstr_tok: ["Utility","class","to","resolve","Placeholder","expressions",".","Placeholder","expression","is","defined","as","a","proto","structure","placeholder_pb2.PlaceholderExpression",".","It","can","be","resolved","with","ResolutionContext","to","a","concrete","value","."]
docstr: Evaluates the index operator.
code: def _resolve_index_operator(
      self, op: placeholder_pb2.IndexOperator
  ) -> Any:
      """Evaluates the index operator."""
      value = self.resolve(op.expression)
      if value is None or not value:
          raise NullDereferenceError(op.expression)
      index_or_key = op.key if op.key else op.index
      try:
          return value[index_or_key]
      except (TypeError, IndexError) as e:
          raise ValueError(
              f"IndexOperator failed to access the given index {index_or_key}."
          ) from e
  
imports: import base64
  import enum
  import functools
  import os
  import re
  from typing import Any, Callable, Dict, List, Set, Union, cast
  from absl import logging
  import attr
  from tfx.dsl.io import fileio
  from tfx.orchestration.portable import data_types
  from tfx.proto.orchestration import placeholder_pb2
  from tfx.types import artifact
  from tfx.types import artifact_utils
  from tfx.types import value_artifact
  from tfx.utils import json_utils
  from tfx.utils import proto_utils
  from google.protobuf import any_pb2
  from google.protobuf import descriptor
  from google.protobuf import json_format
  from google.protobuf import message
  from google.protobuf import text_format
label_desc: Use image node_id 8 for calling the _ExpressionResolver obj's underlying member method code with example usage: obj._resolve_index_operator(op) and returns: value