Prepend a level to a pandas MultiIndex

Question

I have a DataFrame with a MultiIndex created after some grouping:

import numpy as np
import pandas as p
from numpy.random import randn

df = p.DataFrame({
           


        

Answer 1

A nice way to do this in one line using pandas.concat():

import pandas as pd

pd.concat([df], keys=['Foo'], names=['Firstlevel'])

An even shorter way:

pd.concat({'Foo': df}, names=['Firstlevel'])

This can be generalized to many data frames, see the docs.