How to Convert a MultiIndex to type String or List of Strings in Pandas   

To convert Pandas MultiIndex to list of strins or Sting we have several options:

(1) Lambda and custom format

midx.to_series().apply(lambda x: '{0}-{1}-{1}'.format(*x)).values

(2) List comprehension

array(['11-21-21', '11-22-22', '12-21-21', '12-22-22'], dtype=object)

Data

Sample data:

import pandas as pd

df = pd.DataFrame(
    {"Grade": ["A", "B", "A", "C"]},
    index=[
        ["11", "11", "12", "12"],
        ["21", "22", "21", "22"],
        ["31", "32", "33", "34"]
    ]
)

Lambda and custom format

midx.to_series().apply(lambda x: '{0}-{1}-{1}'.format(*x)).values

result:

array(['11-21-21', '11-22-22', '12-21-21', '12-22-22'], dtype=object)

List comprehension

array(['11-21-21', '11-22-22', '12-21-21', '12-22-22'], dtype=object)

result:

['31 21 11', '32 22 11', '33 21 12', '34 22 12']

Flatten MultiIndex

from itertools import starmap

def flat2(midx, sep=''):
    fstr = sep.join(['{}'] * midx.nlevels)
    return pd.Index(starmap(fstr.format, midx))

flat2(midx, sep='_')   

Result:

Index(['11_21_31', '11_22_32', '12_21_33', '12_22_34'], dtype='object')

For more examples and details on MultiIndex flattening please check: How to Flatten a MultiIndex in Pandas

Resource

• How do I convert a MultiIndex to type string