熊猫：即使缺少数据，也为列的每个唯一值创建行

Question

Note : I had difficulty wording the title of my question, so if you can think of something better to help other people with a similar question, please let me know and I will change it.

Current Data

Stored as a Pandas DataFrame

print(df)

week | site | vol
1    | a    | 10
2    | a    | 11
3    | a    | 2
1    | b    | 55
2    | b    | 1
1    | c    | 69
2    | c    | 66
3    | c    | 23

Notice that site b has no data for week 3

Goal

week | site | vol
1    | a    | 10
2    | a    | 11
3    | a    | 2
1    | b    | 55
2    | b    | 1
3    | b    | 0
1    | c    | 69
2    | c    | 66
3    | c    | 23

Essentially, I want to create rows for all of the unique combinations of week and site . If the original data doesn't have a vol for a week-site combo, then it gets a 0 .

Answer 1

Using stack with unstack

df.set_index(['week','site']).unstack('week',fill_value=0).stack().reset_index()
Out[424]: 
  site  week  vol
0    a     1   10
1    a     2   11
2    a     3    2
3    b     1   55
4    b     2    1
5    b     3    0
6    c     1   69
7    c     2   66
8    c     3   23

Answer 2

You can use crosstab and stack :

pd.crosstab(df.site,df.week,df.vol, aggfunc='first').fillna(0).stack().reset_index(name='vol')

Output:

  site  week   vol
0    a     1  10.0
1    a     2  11.0
2    a     3   2.0
3    b     1  55.0
4    b     2   1.0
5    b     3   0.0
6    c     1  69.0
7    c     2  66.0
8    c     3  23.0