Add column values at certain rows pandas

Question

I have a dataframe like this:

x,y,z
1,2,3
2,3,4
1,6,7

And I have a column like this:

a
4
5

Column a should be added when x = 1. So it should look like this:

x, y, z, a
1 2 3 4
2 3 4 NaN
1 6 7 5

How would I do this using pandas

Answer 1

Assign pandas object index will be one of the hidden key , so we should do assign by value only

df.loc[df['x'] == 1, 'a'] = col['a'].values