cv2.createTrackbar(TRACKBAR_NAME, WINDOW_NAME, 0, MAX_VAL, on_trackbar_foo)

cv2.setTrackbarMax(TRACKBAR_NAME, WINDOW_NAME, N)

import cv2
from functools import partial

def on_trackbar_class(value, param):
    param['current_class'] = int(value)
    param['CHANGED_CLASS'] = True       # signal that the class-trackbar was moved

# ...

# this gets the list of classes from the dataset metadata
# CLASSES = dataset.get_classes()
CLASSES = ["head", "left_hand", "right_hand", "right_leg", "left_leg"]

# this data is modified via the trackbar and then used elsewhere
current_class = 0 # this is passed down as a function argument from elsewhere, use '0' here for testing
trackbar_data = {'current_class': current_class}

# create window
WINDOW_NAME = "Navigation controls"
cv2.namedWindow(WINDOW_NAME, cv2.WINDOW_AUTOSIZE)

# create trackbar
TRACKBAR_NAME = 'Class'
max_classes = (len(CLASSES) - 1) if len(CLASSES) > 1 else 1
cv2.createTrackbar(TRACKBAR_NAME, WINDOW_NAME, 0, max_classes, partial(on_trackbar_class, param=trackbar_data))

# the length of the CLASSES might change at some point
# this actually happens elsewhere, but I'm putting this in the same function right here
cv2.setTrackbarMax(TRACKBAR_NAME, WINDOW_NAME, len(CLASSES) - 1) 

# keep the window opened for testing
cv2.waitKey(0)