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

Let's say DataFrame is called df. First copy the y column.

df["z"] = df["y"].copy()

Then set the nan locations of z to the locations in x where the nans are in z.

import numpy as np
df.z[np.isnan(df.z)]=df.x[np.isnan(df.z)]


>>> df 
   x   y   z
0  1 NaN   1
1  2   8   8
2  4  10  10
3  8 NaN   8