interestingly, the example need both 'Panel' and 'ignore_index' from pandas. while 'Panel' was removed since version 0.25.0, 'ignore_index' was added to DataFrame.sort_values() since version 1.0.0 onward. so logically these two requirements can not be fulfilled both no matter what version pandas to be installed. Not sure how the colab setup the dependency to make it work.. I was confused.