Python Group BY Cumsum

Question

I have this DataFrame :

Value  Month
 0       1
 1       2
 8       3
 11      4
 12      5
 17      6
 0       7
 0       8
 0       9
 0       10
 1       11
 2       12
 7       1
 3       2
 1       3
 0       4
 0       5

And i want to create new variable "Cumsum" like this :

Value  Month  Cumsum
 0       1       0
 1       2       1
 8       3       9 
 11      4       20
 12      5       32
 17      6
 0       7
 0       8       ...
 0       9
 0       10
 1       11
 2       12
 7       1       7
 3       2       10
 1       3       11
 0       4       11
 0       5       11

Sorry if my code it is not clean, I failed to include my dataframe ...

My problem is that I do not have only 12 lines (1 line per month) but I have many more lines. By cons I know that my table is tidy and I want to have the cumulated amount until the 12th month and repeat that when the month 1 appears.

Thank you for your help.

Answer 1

Try:

df['Cumsum'] = df.groupby((df.Month == 1).cumsum())['Value'].cumsum()
print(df)

     Value  Month   Cumsum
0      0      1       0
1      1      2       1
2      8      3       9
3     11      4      20
4     12      5      32
5     17      6      49
6      0      7      49 
7      0      8      49
8      0      9      49
9      0     10      49
10     1     11      50
11     2     12      52
12     7      1       7
13     3      2      10
14     1      3      11
15     0      4      11
16     0      5      11