splitting lines, need to add element of same kind

[perfringo]

If anything is to be counted my first thought is to go for Counter which happens to be Python's data type for counting.

Assuming that file data.csv containing sample data one can achieve desired output (in descending order):

from collections import Counter
from datetime import datetime as dt


with open('data.csv', 'r') as f:
    count = Counter()
    for line in f:
        _, marker, qty, item = line.strip().split(',')
        count.update({item: int(qty)})

timestamp = dt.today().date().strftime("%d/%m/%Y")

for item, qty in count.most_common():              # for ascending order: count.most_common()[::-1]
    print(f'{timestamp},{marker},{qty},{item}')     

20/02/2021,P,20,fish
20/02/2021,P,15,dog
20/02/2021,P,10,cat