Forward fill an indexed dataframe, but only forward fill per value in index

Question

Year	Month	score1	score2	score3
2022	Jan	98	NaN	100
	feb	NaN	39	NaN
2021	sept	100	50	NaN
	nov	100	NaN	76
	dec	100	52	NaN

(apologies idk how to make a dataframe in this text editor) I created a pivot table with indexes = ['year','month'] and I want to forward fill just score3, but only within the index. How can I get my dataframe to look like:

Year	Month	score1	score2	score3
2022	Jan	98	NaN	100
	feb	NaN	39	100
2021	sept	100	50	NaN
	nov	100	NaN	76
	dec	100	52	76

Answer 1

You can probably use groupby.ffill:

df['score3'] = df['score3'].groupby(level='Year').ffill()

Or, for a more generic way:

cols = ['score3']
df[cols] = df.groupby(level='Year')[cols].ffill()