Here is my dataframe.
There is a date index and there are 4 symbols for each date.
I want to loop over each date for each symbol.
The ‘quantity’ column is calculated based on the ‘tot_value’ of the previous date. The ‘tot_value’ is computed for a specific date and is common for all symbols. The ‘value’ column varies for each symbol for each date.

It is an issue with the way I am using shift here. It does not reference the previous date value.
Instead it uses the default value of tot_value that I use while populating the dataframe.
However, in the final result the tot_value is getting computed correctly.

I am new to python and would appreciate any help with this loop.

dataframe

Here is my code.

import pandas as pd

# create the dataframe 
data = {'symbol': ['A', 'B', 'C', 'D','A', 'B', 'C', 'D','A', 'B', 'C', 'D','A', 'B', 'C', 'D'],
        'date':['05/06/2024','05/06/2024','05/06/2024','05/06/2024',
                               '05/07/2024','05/07/2024','05/07/2024','05/07/2024',
                               '05/08/2024','05/08/2024','05/08/2024','05/08/2024',
                               '05/09/2024','05/09/2024','05/09/2024','05/09/2024'],
       'tot_value': [1000, 1000, 1000, 1000,1000, 1000, 1000, 1000,1000, 1000, 1000, 1000,1000, 1000, 1000, 1000],
       'mult': [1, 1.1, 1.2, 1.3,1.4, 1.5, 1.6, 1.7,1.8, 1.9, 2, 2.1,2.2, 2.3, 2.4, 2.5],
       'quantity': [0, 0, 0, 0,0, 0, 0, 0,0, 0, 0, 0,0, 0, 0, 0],
       'value': [0, 0, 0, 0,0, 0, 0, 0,0, 0, 0, 0,0, 0, 0, 0],
       }

df = pd.DataFrame(data)
df.set_index(['date'], inplace = True)
symbols = df['symbol'].unique()

# loop by date index and symbol
for ind in df.index.unique():
    for symbol in symbols:
        df['quantity'][ind] = df['tot_value'][ind].shift(1) * df['mult'][ind]
        df['value'][ind] = df['quantity'][ind] * 5

    g = df.groupby('date')['value'].sum()
    df['tot_value'][ind] = g.sum()
    
df

Here is the expected output.
Calculations:

date = 5/6
There is no tot_value for a previous date so the quantity column is NaN. Hence the value is also NaN. And tot_value = default value of 1,000.

date = 5/7
tot_value for previous date = 1,000.
The quantity for this date is based on the tot_value of 1,000.
Once the quantity is calculated the value calc is straightforward.
tot_value for 5/7 = tot_value for 5/6 + sum of value for the 4 symbols on 5/7.
tot_value for 5/7 = 1,000 + sum(7,000 + 7,500 + 8,000 + 8,500) = 32,000

date = 5/8
tot_value for previous date = 32,000.
The quantity for this date is based on the tot_value of 32,000.
Once the quantity is calculated the value calc is straightforward.
tot_value for 5/8 = tot_value for 5/7 + sum of value for the 4 symbols on 5/8.
tot_value for 5/8 = 32,000 + sum(288,000 + 304,000 + 320,000 + 336,000) = 1,280,000.

expected output