Pandas Use Value if Not Null, Else Use Value From Next Column

Question

Given the following dataframe:

import pandas as pd
df = pd.DataFrame({\'COL1\': [\'A\', np.nan,\'A\'], 
                   \'COL2\' : [np.nan,\'A\',\'A\']})
         


        

Answer 1

Using .combine_first, which gives precedence to non-null values in the Series or DataFrame calling it:

import pandas as pd
import numpy as np

df = pd.DataFrame({'COL1': ['A', np.nan,'A'], 
                   'COL2' : [np.nan,'B','B']})

df['COL3'] = df.COL1.combine_first(df.COL2)

Output:

  COL1 COL2 COL3
0    A  NaN    A
1  NaN    B    B
2    A    B    A