Update value in sliced dataframe

[ThomasL]

sw.loc[(sw['x']=='M'), 'v']

This is a pandas series (with one value) and you can not assign this to your cell.
Just make it a value.

df.loc[(df['b'] == 'M') & (df['c'].isnull()), 'c'] = sw.loc[(sw['x']=='M'), 'v'].values
df.loc[(df['b'] == 'B') & (df['c'].isnull()), 'c'] = sw.loc[(sw['x']=='B'), 'v'].values

Output:
	a	b	c
0	A	B	15.2
1	Z	M	1.7
2	A	B	66.6
3	Z	B	16.8
4	Z	M	99.9
5	A	M	99.9
6	Z	B	66.6