code: def test_bam_samples(self):
      reader = sam_reader.SamReader.from_file(
          reads_path=self.bam, ref_path="", options=self.options
      )
      with reader:
          self.assertLen(reader.header.read_groups, 1)
          self.assertEqual(
              reader.header.read_groups[0].sample_id, "NA12878"
          )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from absl.testing import absltest
  from absl.testing import parameterized
  from third_party.nucleus.io import clif_postproc
  from third_party.nucleus.io.python import sam_reader
  from third_party.nucleus.protos import reads_pb2
  from third_party.nucleus.protos import reference_pb2
  from third_party.nucleus.testing import test_utils
  from third_party.nucleus.util import ranges
label_desc: Use image node_id 4 for calling the SamReaderTest obj's underlying member method code with example usage: obj.test_bam_samples() without return types