Trying some alternatives.
Modified one row of my csv-dataset to have an empty class label and set all bounding box vertices columns to string to allow passing an empty string.
  # TODO: Remove this since it's only here to mockup a row where trying to insert a null-class example
  #       This might fail since documentation suggest this is not possible
  mycsv['xmin'] = mycsv['xmin'].astype(str)
  mycsv['ymin'] = mycsv['ymin'].astype(str)
  mycsv['xmax'] = mycsv['xmax'].astype(str)
  mycsv['ymax'] = mycsv['ymax'].astype(str)
  mycsv.at[0,'class'] = '' 
  mycsv.at[0,'xmin'] = ''
  mycsv.at[0,'ymin'] = ''
  mycsv.at[0,'xmax'] = ''
  mycsv.at[0,'ymax'] = ''


Fails during load:
/usr/local/lib/python3.7/dist-packages/tensorflow_examples/lite/model_maker/core/data_util/object_detector_dataloader_util.py in _get_xml_dict_from_csv_lines(images_dir, image_filename, lines)
    335   for line in lines:
    336     label = line[2].strip()
--> 337     xmin, ymin = float(line[3]) * width, float(line[4]) * height
    338     xmax, ymax = float(line[7]) * width, float(line[8]) * height
    339     obj = {

ValueError: could not convert string to float: 

Leaving bbox columns as float and inserting None yeilds the same result
  mycsv.at[0,'class'] = '' 
  mycsv.at[0,'xmin'] = None
  mycsv.at[0,'ymin'] = None
  mycsv.at[0,'xmax'] = None
  mycsv.at[0,'ymax'] = None


Columns xmin, xmax, ymin and ymax becomes strings:

Expand Table


split
filename
class
xmin
ymin
empty1
empty2
xmax
ymax
empty3
empty4




TEST
file1.jpg











TEST
file2.jpg
red
0.27
0.0


1.0
1.0




TEST
file3.jpg
blue
0.37
0.7866666666666666


1.0
1.0




TEST
file4.jpg
blue
0.426
0.18


0.67
0.59






Fails during load again:
/usr/local/lib/python3.7/dist-packages/tensorflow_examples/lite/model_maker/core/data_util/object_detector_dataloader_util.py in _get_xml_dict_from_csv_lines(images_dir, image_filename, lines)
    335   for line in lines:
    336     label = line[2].strip()
--> 337     xmin, ymin = float(line[3]) * width, float(line[4]) * height
    338     xmax, ymax = float(line[7]) * width, float(line[8]) * height
    339     obj = {

ValueError: could not convert string to float: