Pandas Groupby then cumulative percentage of the groups

So my Dataset looks like this, I want to group my dataset by TSI column first and then find cum% of ONS for each grouped TSI

How do I find cumulative percentage of each group so i get

a = df.groupby(['TSI'])['Final ONS'].cumsum()
b = df.groupby(['TSI'])['Final ONS'].sum()
g = df.groupby(['TSI'])['Final 
ONS'].cumsum(axis=0)/df.groupby(['TSI'])['Final ONS'].sum()
g = pd.DataFrame(g)
g

This code gives me nan values

Here's one approach:

(i) groupby "F_TSI" and find cumsum of "F_ONS" for each group

(ii) groupby "F_TSI" and transform the sum of "F_ONS" for each group

(iii) divide (i) by (ii) for the percentages

gb_obj = df.groupby('F_TSI')['F_ONS']
df['Cum%ONS'] = gb_obj.cumsum() / gb_obj.transform('sum') * 100

Output:

   F_TSI  F_ONS     Cum%ONS
0  A1020    0.5    7.042254
1  A1020    1.3   25.352113
2  A1020    2.2   56.338028
3  A1020    3.1  100.000000
4  B1020    0.5   26.315789
5  B1020    1.4  100.000000
6  D1020    0.5   26.315789
7  D1020    1.4  100.000000