How to calculate inverse cumsum in pandas

Question

I am trying to find a way to calculate an inverse cumsum for pandas. This means applying cumsum but from bottom to top. The problem I\'m facing is, I\'m trying to f

Answer 1

Invert the row order of the DataFrame prior to grouping so that the cumsum is calculated in reverse order within each month.

df['inv_workable_day'] = df[::-1].groupby('month')['flag_workable'].cumsum()
df['workable_day'] = df.groupby('month')['flag_workable'].cumsum()

#         Date  flag_workable  month  inv_workable_day  workable_day
#1  2019-01-02           True      1               5.0           1.0
#2  2019-01-03           True      1               4.0           2.0
#3  2019-01-04           True      1               3.0           3.0
#6  2019-01-07           True      1               2.0           4.0
#7  2019-01-08           True      1               1.0           5.0
#8  2019-02-01           True      2               1.0           1.0

Answer 2

Solution

Whichever column you want to apply cumsum to you have two options:
1. Order descending a copy of that column by index, followed by cumsum and then order ascending by index. Finally assign it back to the data frame column.

2. Use numpy:

import numpy as np

array = df.column_data.to_numpy()    
array = np.flip(array)  # to flip the order 
array = numpy.cumsum(array)    
array =  numpy.flip(array)  # to flip back to original order    
df.column_data_cumsum = array