code: def get_classes(self):
      return sorted(
          set(
              [
                  b.get("label", self._default_class)
                  for r in self.annotations
                  for b in r.get(self._objects_key, [])
              ]
          )
      )
  
file_path: luminoth/luminoth/tools/dataset/readers/object_detection/flat_reader.py
imports: import json
  import os
  import six
  import tensorflow
  from PIL import Image
  from luminoth.tools.dataset.readers import InvalidDataDirectory
  from luminoth.tools.dataset.readers.object_detection import ObjectDetectionReader
  from luminoth.utils.dataset import read_image
label_desc: Use image node_id 3 for calling the FlatReader obj's underlying member method code with example usage: obj.get_classes() and returns: sorted