count occurrences of one variable grouped by another python

1>>> data = pd.DataFrame({'user_id' : ['a1', 'a1', 'a1', 'a2','a2','a2','a3','a3','a3'], 'product_id' : ['p1','p1','p2','p1','p1','p1','p2','p2','p3']})
2>>> count_series = data.groupby(['user_id', 'product_id']).size()
3>>> count_series
4user_id  product_id
5a1       p1            2
6         p2            1
7a2       p1            3
8a3       p2            2
9         p3            1
10dtype: int64
11>>> new_df = count_series.to_frame(name = 'size').reset_index()
12>>> new_df
13  user_id product_id  size
140      a1         p1     2
151      a1         p2     1
162      a2         p1     3
173      a3         p2     2
184      a3         p3     1
19>>> new_df['size']
200    2
211    1
222    3
233    2
244    1
25Name: size, dtype: int64