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

If we mod your df slightly then you will see that this works and in fact will work for any number of columns so long as there is a single valid value:

In [5]:
df = pd.DataFrame({'COL1': ['B', np.nan,'B'], 
                   'COL2' : [np.nan,'A','A']})
df

Out[5]:
  COL1 COL2
0    B  NaN
1  NaN    A
2    B    A

In [6]:    
df.apply(lambda x: x[x.first_valid_index()], axis=1)

Out[6]:
0    B
1    A
2    B
dtype: object

first_valid_index will return the index value (in this case column) that contains the first non-NaN value:

In [7]:
df.apply(lambda x: x.first_valid_index(), axis=1)

Out[7]:
0    COL1
1    COL2
2    COL1
dtype: object

So we can use this to index into the series