# Cumulative weights with monthly rebalancing python

129
March 25, 2021, at 06:40 AM

I have real struggle here:

I have a dataframe with columns like this excel file, where PERMNO is each stock and weights are already calculated and sum up to 1 each day:

What i want to achieve is like the left column in this example with just two stocks, but for all stocks.

[]

If we look at the problem with only two stocks, what i want to do is achieved like this:

Wt = Wt-1*(1+rt)/sum(wt all stocks this t)

Stock 1 has weight 0.5 and return 0.01, stock 2 has weight 0.5 and return 0,03 the first day of the month. Hence, the weight of the two stocks this day is:

Day 1: stock 1: 0.51.01/(0.51.01+0.51.03) = 0.4951 stock 2:0.51.03/(0.51.01+0.51.03) = 0,5049

next day like this: Day 2:

stock 1: 0.49511.02/(0.49511.02+0,50491.01) = 0.4975 stock 2: 0,50491.01/(0.49511.02+0,50491.01) = 0,5024

and this shall continue until the end of the month. After that, the first return for each stock is used to start the same process again. The idea is to do this with a dataframe like the first picture.

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