缺少行的 Pandas Groupby

Question

I have a dataframe that looks like this (actual df contains million rows), consisting of data for Week 1,2 and 3. If the Qty is 0 for that week, then the rows are not present. For example, Product A qty=0 for Week 2, thus there is no Week 2 row shown in the dataframe.

Product	Week	Qty
A	1	10
A	1	10
A	3	10
B	2	10
B	2	10

I want to groupby into this dataframe showing the qty for each week including when Qty=0. How to achieve this? Thank you.

Product	Week	Qty
A	1	20
A	2	0
A	3	10
B	1	0
B	2	20
B	3	0

Answer 1

#Groupby and sum
df1=df.groupby(['Product','Week'])['Qty'].sum().to_frame()

#reindex, multiindex
df1.reindex( pd.MultiIndex.from_product([df1.index.levels[0], 
df['Week'].unique()], names=['Product', 'Week']),fill_value=0).reset_index()





    Product  Week  Qty
0       A     1   20
1       A     3   10
2       A     2    0
3       B     1    0
4       B     3    0
5       B     2   20