python - How to implement sql coalesce in pandas

Question

I have a data frame like

df = pd.DataFrame({"A":[1,2,np.nan],"B":[np.nan,10,np.nan], "C":[5,10,7]})
     A     B   C
0  1.0   NaN   5
1  2.0  10.0  10
2  NaN   NaN   7 

I want to add a new column 'D'. Expected output is

     A     B   C    D
0  1.0   NaN   5    1.0
1  2.0  10.0  10    2.0
2  NaN   NaN   7    7.0

Thanks in advance!

Answer 1

Another way is to explicitly fill column D with A,B,C in that order.

df['D'] = np.nan
df['D'] = df.D.fillna(df.A).fillna(df.B).fillna(df.C)