code: def test_make_population_vcf_readers_with_multiple_vcfs(self):
      filenames = [testdata.AF_VCF_CHR20, testdata.AF_VCF_CHR21]
  
      output = allele_frequency.make_population_vcf_readers(filenames)
  
      self.assertIsInstance(output["chr20"], vcf.VcfReader)
      self.assertIsInstance(output["chr21"], vcf.VcfReader)
      self.assertEqual(next(output["chr20"]).reference_name, "chr20")
      self.assertEqual(next(output["chr21"]).reference_name, "chr21")
      # Check that chr22 has no reader rather than outputting another reader for
      # a different chromosome.
      self.assertIsNone(output["chr22"])
  
file_path: deepvariant/deepvariant/allele_frequency_test.py
imports: from absl.testing import absltest
  from absl.testing import parameterized
  from third_party.nucleus.io import fasta
  from third_party.nucleus.io import vcf
  from third_party.nucleus.protos import variants_pb2
  from deepvariant import allele_frequency
  from deepvariant import testdata
  from deepvariant.protos import deepvariant_pb2
label_desc: Use image node_id 4 for calling the AlleleFrequencyTest obj's underlying member method code with example usage: obj.test_make_population_vcf_readers_with_multiple_vcfs() without return types