import csv
 import time
 import datetime
 import re
 from operator import itemgetter
 from multiprocessing import Pool
 import itertools
 
 def chunks(l,n):
  """Divide a list of nodes `l` in `n` chunks"""
  l_c = iter(l)
  while 1:
  x = tuple(itertools.islice(l_c,n))
  if not x:
  return
  yield x
 
 def csv2nodes(r):
  strptime = time.strptime
  mktime = time.mktime
  l = []
  ppl = set()
  pattern = re.compile(r"""[A-Za-z0-9"/]+?(?=[,\n])""")
  for row in r:
  with pattern.findall(row) as f:
  cell = int(f[3])
  id = int(f[2])
  st = mktime(strptime(f[0],'%d/%m/%Y'))
  ed = mktime(strptime(f[1],'%d/%m/%Y'))
  # collect list
  l.append([(id,cell,{1:st,2: ed})])
  # collect separate sets
  ppl.add(id)
  return (l,ppl)
 
 def csv2graph(source):
  MG=nx.MultiGraph()
  # Remember that I use integers for edge attributes, to save space! Dic above.
  # start: 1
  # end: 2
  p = Pool()
  node_divisor = len(p._pool)
  node_chunks = list(chunks(source,int(len(source)/int(node_divisor))))
  num_chunks = len(node_chunks)
  pedgelists = p.map(csv2nodes,
  node_chunks)
  ll = []
  ppl = set()
  for l in pedgelists:
  ll.append(l[0])
  ppl.update(l[1])
  MG.add_edges_from(ll)
  return (MG,ppl)
 
 with open('/Users/laszlosandor/Dropbox/peers_prisons/python/codetenus_test.txt','r') as source:
  r = source.readlines()
  MG,ppl = csv2graph(r)

Traceback (most recent call last):
  File "parser5_nodots_parallel.py", line 256, in <module>
  MG,ppl = csv2graph(r)
  File "parser5_nodots_parallel.py", line 245, in csv2graph
  node_chunks)
  File "/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/multiprocessing/pool.py", line 251, in map
  return self.map_async(func, iterable, chunksize).get()
  File "/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/multiprocessing/pool.py", line 552, in get
  raise self._value
 AttributeError: __exit__