Pandas fillna() 不将一列中的值应用到整个 dataframe

Question

Please explain, why this won't work (hc is pandas dataframe on below example):

import pandas as pd
import numpy as np

hc = pd.DataFrame([['Adolf', np.nan],
                  ['Hans', 'Johan']],
                  columns=('First Name', 'Second Name'))

hc.fillna(value=hc["First Name"])

 First Name Second Name 0 Adolf NaN 1 Hans Johan

hc[['First Name','Second Name']].fillna(value=hc["First Name"])

 First Name Second Name 0 Adolf NaN 1 Hans Johan

Using other column's value works only for one column like that:

hc['Second Name'].fillna(value=hc["First Name"],inplace=True)
hc

 First Name Second Name 0 Adolf Adolf 1 Hans Johan

I wish to apply a value (mask value) from one column to NaN-s in the entire dataframe.

Answer 1

IIUC, you want to fillna with a Series as reference, per row.

hc = hc.fillna(hc['First Name'])

This is not currently supported on the columns for all pandas version (it is in the latest), but you can cheat using a double transpose to perform the operation on the rows:

hc = hc.T.fillna(hc['First Name']).T

output:

  First Name Second Name
0      Adolf       Adolf
1       Hans       Johan