python - Filling a row in a csv file according -

here csv file want update python. has 3 columns:

col1 = [1,2,3,4,5,6,7,8,9,10]  col2 = [a, a, a, b, c, c, c, c, a, a]  col3 = [] 

for each cell in col3 fill id number change when content of col2 changes.

in example col3 = [1,1,1,2,3,3,3,3,4,4]

anyone show me way ?

in python 3:

from itertools import groupby operator import itemgetter import csv  open('f1.csv', newline='') f_in, open('f2.csv', 'w', newline='') f_out:     reader = csv.reader(f_in)     writer = csv.writer(f_out)     i, (key, group) in enumerate(groupby(reader, key=itemgetter(1)), start=1):         row in group:             writer.writerow(row + [i]) 

if using python 2.7, change with-statement to

with open('f1.csv', 'rb') f_in, open('f2.csv', 'wb') f_out: