On a file, "with file:" fails if it's used a second time:
---
fp = open('/etc/issue')
with fp:
    print("first")
with fp:
    print("second")
---

fails with "ValueError: I/O operation on closed file", because file.__enter__() raises this exception if the file is closed.

I propose to have the same behavior on multiprocessing.Pool.__enter__() to detect when the multiprocessing API is misused.

Anyway, after the first "with pool:" block, the pool becomes unusable to schedule now tasks: apply() raise ValueError("Pool not running") in that case for example.

Currently, the error only occurs when apply() is called:
---
import multiprocessing

def the_test():
    pool = multiprocessing.Pool(1)
    with pool:
        print(pool.apply(int, (2,)))
    with pool:
        print(pool.apply(int, (3,))) # <-- raise here

the_test()
---

I would prefer to get an error on at the second "with pool:" line.

New changeset 08c2ba0717089662132af69bf5948d82277a8a69 by Victor Stinner in branch 'master':
bpo-35477: multiprocessing.Pool.__enter__() fails if called twice (GH-11134)
https://github.com/python/cpython/commit/08c2ba0717089662132af69bf5948d82277a8a69