How do you update the levels of a pandas MultiIndex after slicing its DataFrame?

Question

I have a Dataframe with a pandas MultiIndex:

In [1]: import pandas as pd
In [2]: multi_index = pd.MultiIndex.from_product([[\'CAN\',\'USA\'],[\'total\']],nam         


        

Answer 1

This is something that has bitten me before. Dropping columns or rows does NOT change the underlying MultiIndex, for performance and philosophical reasons, and this is officially not considered a bug (read more here). The short answer is that the developers say "that's not what the MultiIndex is for". If you need a list of the contents of a MultiIndex level after modification, for example for iteration or to check to see if something is included, you can use:

df.index.get_level_values()

This returns the current active values within that index level.

So I guess the "trick" here is that the API native way to do it is to use get_level_values instead of just .index or .columns