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

To flatten a MultiIndex inside a chain of other DataFrame methods, define a function like this:

def flatten_index(df):
  df_copy = df.copy()
  df_copy.columns = ['_'.join(col).rstrip('_') for col in df_copy.columns.values]
  return df_copy.reset_index()

Then use the pipe method to apply this function in the chain of DataFrame methods, after groupby and agg but before any other methods in the chain:

my_df \
  .groupby('group') \
  .agg({'value': ['count']}) \
  .pipe(flatten_index) \
  .sort_values('value_count')