I have the same warning, and I solve it by ''from future import division ‘’. because in python 2.*,     1/2  =  0.
so in the function" warn_imbalance"
def warn_imbalance(get_prop):
values = [get_prop(props) for props in dev_props]
min_pos, min_val = min(enumerate(values), key=operator.itemgetter(1))
max_pos, max_val = max(enumerate(values), key=operator.itemgetter(1))
if min_val / max_val < 0.75:
warnings.warn(imbalance_warn.format(device_ids[min_pos], device_ids[max_pos]))
return True
return False
min_val / max_val is directly zero