class_docstr: Test-only, not private combine_fn.
class_docstr_tok: ["Test-only",",","not","private","combine_fn","."]
code: def extract_private_output(self, accumulator, budget, params):
      return accumulator
  
imports: import unittest
  import apache_beam
  from apache_beam.runners.portability import fn_api_runner
  from apache_beam import pvalue
  from unittest.mock import patch
  import apache_beam.testing.util
  from apache_beam.testing.test_pipeline import TestPipeline
  import pipeline_dp
  from pipeline_dp import private_beam
  from pipeline_dp import aggregate_params, budget_accounting
label_desc: Use image node_id 4 for calling the SumCombineFn obj's underlying member method code with example usage: obj.extract_private_output(accumulator, budget, params) and returns: accumulator