[python] How to troubleshoot an "AttributeError: __exit__" in multiproccesing in Python?

I tried to rewrite some csv-reading code to be able to run it on multiple cores in Python 3.2.2. I tried to use the Pool object of multiprocessing, which I adapted from working examples (and already worked for me for another part of my project). I ran into an error message I found hard to decipher and troubleshoot.

The error:

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__

The relevant code:

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)

What's a good way to troubleshoot this?

The error also happens when trying to use the

with multiprocessing.Pool() as pool:
   # ...

with a Python version that is too old (like Python 2.X) and does not support using with together with multiprocessing pools.

(See this answer https://stackoverflow.com/a/25968716/1426569 to another question for more details)

The reason behind this error is : Flask app is already running, hasn't shut down and in middle of that we try to start another instance by: with app.app_context(): #Code Before we use this with statement we need to make sure that scope of the previous running app is closed.

It is not the asker's problem in this instance but the first troubleshooting step for a generic "AttributeError: __exit__" should be making sure the brackets are there, e.g.

with SomeContextManager() as foo:
    #works because a new object is referenced...

not

with SomeContextManager as foo:
    #AttributeError because the class is referenced

Catches me out from time to time and I end up here -__-