How to pass another entire column as argument to pandas fillna()

Question

I would like to fill missing values in one column with values from another column, using fillna method.

(I read that looping through each row would be

Answer 1

You can provide this column to fillna (see docs), it will use those values on matching indexes to fill:

In [17]: df['Cat1'].fillna(df['Cat2'])
Out[17]:
0    cat
1    dog
2    cat
3    ant
Name: Cat1, dtype: object