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

Just use the value parameter instead of method:

In [20]: df
Out[20]:
  Cat1      Cat2  Day
0  cat     mouse    1
1  dog  elephant    2
2  cat     giraf    3
3  NaN       ant    4

In [21]: df.Cat1 = df.Cat1.fillna(value=df.Cat2)

In [22]: df
Out[22]:
  Cat1      Cat2  Day
0  cat     mouse    1
1  dog  elephant    2
2  cat     giraf    3
3  ant       ant    4