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

The update method does almost exactly this. The only caveat is that update will do so in place so you must first create a copy:

df['z'] = df.x.copy()
df.z.update(df.y)

In the above example you start with x and replace each value with the corresponding value from y, as long as the new value is not NaN.