Python Dataframe issue of filling NaN with column name

Question

I want my dataframe to fill NaN in a column with other column values. But, my dataframe fills NaN with column name, not its values. It is strange. I have no idea. why.

df=
                          colA    colB
                                       
2018-11-20 06:57:30       NaN  20.60646
2018-11-20 06:57:45       NaN  23.96100
2018-11-20 06:58:00       NaN  27.31554
2018-11-20 06:58:15       NaN  30.67008
2019-05-22 18:45:00  20.07324  20.76620
2019-05-22 18:45:15  19.92117  20.44672
2019-05-22 18:45:30  19.61703  20.12724

df['colA'].fillna('colB',inplace=True)

print(df)

df=
                          colA    colB
                                       
2018-11-20 06:57:30       colB 20.60646
2018-11-20 06:57:45       colB 23.96100
2018-11-20 06:58:00       colB 27.31554
2018-11-20 06:58:15       colB 30.67008
2019-05-22 18:45:00  20.07324  20.76620
2019-05-22 18:45:15  19.92117  20.44672
2019-05-22 18:45:30  19.61703  20.12724

Answer 1

You need '.' operator or the column name as a string to the indexing operator.

df['colA'].fillna(df.colB,inplace=True)

df['colA'].fillna(df['colB'],inplace=True)