memory usage multi value hash

[christian]

Hello,

i'm not very experienced in python. Is there a way doing below more
memory efficient and maybe faster.
I import a  2-column file and  then concat for every unique value in
the first column ( key) the value from the second
columns.

So The ouptut is something like that.
A,1,2,3
B,3,4
C,9,10,11,12,90,34,322,21


Thanks for advance & regards,
Christian


import csv
import random
import sys
from itertools import groupby
from operator import itemgetter

f=csv.reader(open(sys.argv[1]),delimiter=';')
z=[[i[0],i[1]] for i in f]
z.sort(key=itemgetter(0))
mydict = dict((k,','.join(map(itemgetter(1), it)))
           for k, it in groupby(z, itemgetter(0)))
del(z)

f = open(sys.argv[2], 'w')
for k,v in mydict.iteritems():
    f.write(v + "\n")

f.close()