How to set flags as -1 for n-1 rows before a cell in python(to drop)?

Question

How do I set the n-1 values before a flag value to -1 as I need to drop the n-1 values before the flag value for example, if the flag value is 4, I need to set the previous 3 rows to -1.

Example Table

UID	exclusion_flag
1BOP2UC-1	0
1BOP2UC-2	0
1BOP2UC-3	0
1BOP2UC-4	4
1BOP2UC-5	0
1BOP2UC-6	0
1BOP2UC-7	0
1BOP2UC-8	2
1BOP2UC-9	0
1BOP2UD-1	0
1BOP2UD-2	0
1BOP2UD-3	0
1BOP2UD-4	0
1BOP2UD-5	4

Required Solution

UID	exclusion_flag
1BOP2UC-1	-1
1BOP2UC-2	-1
1BOP2UC-3	-1
1BOP2UC-4	4
1BOP2UC-5	0
1BOP2UC-6	0
1BOP2UC-7	-1
1BOP2UC-8	2
1BOP2UC-9	0
1BOP2UD-1	0
1BOP2UD-2	-1
1BOP2UD-3	-1
1BOP2UD-4	-1
1BOP2UD-5	4

Answer 1

I don't know whether question is still actual, but this is my solution:

1. I initialize mem variable wich represents counter.
2. I iterate thru list backwards using reversed function.
3. If actual value is greater than zero, I want to overwrite mem with actual value otherwise, mem -1. I don't want counter to be negative, therefore max(0, mem-1).
4. If mem is greater then zero, it means we are still counting rows, mark row to -1.
5. If mem equals to x this is row, where our counter sequence started, and we want to keep x besides out.
6. I return the out_arr again in correct order.

from typing import List
import pandas as pd

dummy_col = [0, 0, 0, 0, 4, 0, 0, 0, 2]
df = pd.DataFrame(dict(col1=dummy_col))


def fun(arr: List[int]) -> List[int]:
    mem = 0
    out_arr = []
    for x in reversed(arr):
        mem = x if x > 0 else max(0, mem - 1)
        out = -1 if mem > 0 else 0
        out = x if mem == x else out
        out_arr += [out]
    return list(reversed(out_arr))


df['final_col'] = fun(arr=df['col1'].to_list())
print(df)

OUTPUT:

   col1  final_col
0     0          0
1     0         -1
2     0         -1
3     0         -1
4     4          4
5     0          0
6     0          0
7     0         -1
8     2          2