code: def testUnsupportedOperator(self):
      predicate = _TEST_CHANNEL.future()[0].b64encode() == "test_str"
      channel_to_key_map = {
          _TEST_CHANNEL: "key",
      }
      placeholder_pb = channel_utils.encode_placeholder_with_channels(
          predicate, lambda c: channel_to_key_map[c]
      )
      with self.assertRaisesRegex(
          ValueError,
          "Got unsupported placeholder operator base64_encode_op.",
      ):
          compiler_utils.placeholder_to_cel(placeholder_pb)
  
imports: import os
  from absl.testing import parameterized
  from kfp.pipeline_spec import pipeline_spec_pb2
  import tensorflow
  from tfx.dsl.io import fileio
  from tfx.orchestration import data_types
  from tfx.orchestration.kubeflow.v2 import compiler_utils
  from tfx.types import artifact
  from tfx.types import channel
  from tfx.types import channel_utils
  from tfx.types import standard_artifacts
  from tfx.types.experimental import simple_artifacts
  import yaml
  from google.protobuf import text_format
label_desc: Use image node_id 6 for calling the PlaceholderToCELTest obj's underlying member method code with example usage: obj.testUnsupportedOperator() without return types