csv writer not closing file

im reading a csv file and then writing a new one:

import csv

with open('thefile.csv', 'rb') as f:
  data = list(csv.reader(f))

import collections
counter = collections.defaultdict(int)
for row in data:
    counter[row[11]] += 1

writer = csv.writer(open('/pythonwork/thefile_subset1.csv', 'w'))
for row in data:
    if counter[row[11]] >= 500:
       writer.writerow(row)

for some reason i cannot get the csv.writer to close the file. when i open the file it opens it as READ ONLY because it says that is still open.

how do i close thefile_subset1.csv after i am done with it?