Weight Distribution

[Pedroski55]

Second Step: Since insurance in Level_3 group does not have any value in child_weight,
parent_weight value gets divided among all rows which has same Level_2.

All 5 rows have Financial in Level_2, how is the above supposed to work?
Did you really check the data?

import pandas as pd

cols = ['Security', 'CompRating', 'Level_3', 'Level_2', 'parent_weight', 'child_weight']

# revised data
datastring = """A A1 Banking Financial 4.486 3.522
B BBB3 Banking Financial 0.445 0.559
C A1 Financial_Services Financial 0.821 0.994
D A3 Financial_Services Financial 2.744 3.445
E A2 Financial_Services Financial 1.919 0.456
F A1 Insurance Financial 0.393 NULL"""

lists = [s.split() for s in datastring.split('\n')]
df = pd.DataFrame(lists, columns=cols)
# set the number of columns to show 
pd.set_option('display.max_columns', 10)
# add a column with a default value of 0 or set all values to 0 if df['Totals'] exists
df['Totals'] = 0
# the numbers are strings, make them floats
df['parent_weight'] = pd.to_numeric(df.parent_weight, errors='coerce')
df['child_weight'] = pd.to_numeric(df.child_weight, errors='coerce')

# calculate totals and write to Totals for Banking and Financial_Services
for index in df.index:
    # step 1.1 make sums
    if df['Level_3'][index] == 'Banking':
        sumparent = df.loc[df['Level_3'] == 'Banking', 'parent_weight'].sum()
        sumchild = df.loc[df['Level_3'] == 'Banking', 'child_weight'].sum()
        total = df['Totals'][index] + (df['child_weight'][index] * sumparent/sumchild)
        df.at[index, 'Totals'] = total
    elif df['Level_3'][index] == 'Financial_Services':
        sumparent = df.loc[df['Level_3'] == 'Financial_Services', 'parent_weight'].sum()
        sumchild = df.loc[df['Level_3'] == 'Financial_Services', 'child_weight'].sum()
        total = df['Totals'][index] + (df['child_weight'][index] * sumparent/sumchild)
        df.at[index, 'Totals'] = total

Output:
  Security CompRating             Level_3    Level_2  parent_weight  \
0        A         A1             Banking  Financial          4.486   
1        B       BBB3             Banking  Financial          0.445   
2        C         A1  Financial_Services  Financial          0.821   
3        D         A3  Financial_Services  Financial          2.744   
4        E         A2  Financial_Services  Financial          1.919   
5        F         A1           Insurance  Financial          0.393   

   child_weight    Totals  
0         3.522  4.255570  
1         0.559  0.675430  
2         0.994  1.113605  
3         3.445  3.859526  
4         0.456  0.510869  
5           NaN  0.000000  

Sorry, but I don't understand what you want to do with the row Insurance, please explain.