How to troubleshoot an “AttributeError: __exit__” in multiproccesing in Python?

Question

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 adap

Answer 1

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)

Answer 2

The problem is in this line:

with pattern.findall(row) as f:

You are using the with statement. It requires an object with __enter__ and __exit__ methods. But pattern.findall returns a list, with tries to store the __exit__ method, but it can't find it, and raises an error. Just use

f = pattern.findall(row)

instead.

Answer 3

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 -__-

Answer 4

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.