def image_path_from_index(self, index):
        """
        Construct an image path from the image's "index" identifier.
        """
        # Example image path for index=119993:
        #   images/train2014/COCO_train2014_000000119993.jpg
        file_name = ('COCO_' + self._data_name + '_' +
                     str(index).zfill(12) + '.jpg')
        image_path = osp.join(self._data_path, 'images',
                              self._data_name, file_name)
        assert osp.exists(image_path), \
                'Path does not exist: {}'.format(image_path)
        return image_path

        Construct an image path from the image's "index" identifier.
        
lib/datasets/coco.py