Pandas - How to flatten a hierarchical index in columns

Question

I have a data frame with a hierarchical index in axis 1 (columns) (from a groupby.agg operation):

     USAF   WBAN  year  month  day  s_PC  s_CL         


        

Answer 1

A general solution that handles multiple levels and mixed types:

df.columns = ['_'.join(tuple(map(str, t))) for t in df.columns.values]