filling a NaN column by another column content in pandas dataframe

Question

I have a dataframe with a column having lots of NaN values. I need to updates these values by another column values when NaN is in the column.

    One    Two
0    10    21

1    11    NaN

2    12    25

I want a result like this

    One    Two

0    10    21

1    11    **11**

2    12    25

Please help me for the synthaxe of slicing in the column two and have Nafill detection to copy the value from column One.

This is the code to create the 1rst matrix :

 df = pd.DataFrame({'One' : [10, 11, 12], 'Two' : [21, np.NAN, 25]})

Answer 1

You can use fillna using that column:

In [4]: df['Two'].fillna(df['One'])
Out[4]:
0    21
1    11
2    25
Name: Two, dtype: float64

So to actually change the column: df['Two'] = df['Two'].fillna(df['One'])