pandas – creating new rows of combination with value of 0

I have a pandas dataframe like

user_id	music_id	has_rating
A	a	1
B	b	1

and I would like to automatically add new rows for each of user_id & music_id for those users haven’t rated, like

user_id	music_id	has_rating
A	a	1
A	b	0
B	a	0
B	b	1

for each of user_id and music_id combination pairs those are not existing in my Pandas dataframe yet.

is there any way to append such rows automatically like this?

Answer

You can use a temporary reshape with pivot_table and fill_value=0 to fill the missing values with 0:

(df.pivot_table(index='user_id', columns='music_id',
                values='has_rating', fill_value=0)
   .stack().reset_index(name='has_rating')
 )

Output:

  user_id music_id  has_rating
0       A        a           1
1       A        b           0
2       B        a           0
3       B        b           1