Adding specific fields from a csv

You can use an OrderedDict storing the names as values and updating the count as you go:

import csv
from collections import OrderedDict

od = OrderedDict()

with open("test.txt") as f:
    r = csv.reader(f)
    head = next(r)
    for name,val in r:
        od.setdefault(name, 0)
        od[name]  += int(val)

print(od)
OrderedDict([('[email protected]', 25), ('[email protected]', 8), ('[email protected]', 2)])

To update the original file you can write to a NamedTemporaryFile then use shutil.move to replace the original after you have written the rows with writerows using the od.items:

import csv
from collections import OrderedDict
from shutil import move
from tempfile import NamedTemporaryFile
od = OrderedDict()

with open("test.txt") as f, NamedTemporaryFile(dir=".",delete=False) as out:
    r = csv.reader(f)
    wr = csv.writer(out)
    head = next(r)
    wr.writerow(head)
    for name,val in r:
        od.setdefault(name, 0)
        od[name]  += int(val)
    wr.writerows(od.iteritems())


move(out.name,"test.txt")

Output:

E-mail,M-count
[email protected],25
[email protected],8
[email protected],2

If you don't care about order use a defaultdict instead:

import csv

from collections import defaultdict
from shutil import move
from tempfile import NamedTemporaryFile
od = defaultdict(int)

with open("test.txt") as f, NamedTemporaryFile(dir=".",delete=False) as out:
    r = csv.reader(f)
    wr = csv.writer(out)
    head = next(r)
    wr.writerow(head)
    for name,val in r:
        od[name]  += int(val)
    wr.writerows(od.iteritems())

import csv

ifile = open('sample.csv', 'rb')
csv_reader = csv.reader(ifile)

d = {}
for row in csv_reader:
    d[row[0]] = int(row[1]) if d.get(row[0], None) is None else d[row[0]] + int(row[1])
from pprint import pprint
pprint(d)