Fillna based on another column

Question

I have a dataframe, how to fillna col2 based on col1? Here fillna with 10, I have multiple col1 values but each one only has 1 corresponding col2 value or it is nan.

col1 col2
 A    Nan
 A    10   

Answer 1

You can try with fillna by groupby and transform

df.col2.fillna(df.groupby('col1').col2.transform('first'), inplace=True)