Python: create a new column from existing columns

Question

I am trying to create a new column based on both columns. Say I want to create a new column z, and it should be the value of y when it is not missing and be the value of x when

Answer 1

Use np.where:

In [3]:

df['z'] = np.where(df['y'].isnull(), df['x'], df['y'])
df
Out[3]:
   x   y   z
0  1 NaN   1
1  2   8   8
2  4  10  10
3  8 NaN   8

Here it uses the boolean condition and if true returns df['x'] else df['y']