how can I write my function output on CSV file

[buran]

you need to reset l2 for every row

import csv
from statistics import mean
   
with open ('grades.csv', 'r') as input_file:
    reader=csv.reader(input_file)
    data = []
    for row in reader:
        l2 = []
        name = row[0]
        grades=[int(num) for num in row[1:]] # alternatively use: grades = map(int, row[1:])
        l2.append(name)
        l2.append(float(mean(grades)))
        data.append(l2)
        # or just as a single line
        # data.append([row[0], float(mean(grades))])
 
with open('grades.csv', 'w', newline='') as output_file_name:
    writer = csv.writer(output_file_name)
    writer.writerows(data)