with 关键字在打开文件时有效，但在调用函数时无效

Question

我正在试验多处理模块并从this 页面复制示例代码。这是一个例子：

#!/usr/bin/python
from multiprocessing import Pool
from time import sleep

def f(x):
    return x*x

if __name__ == '__main__':
    # start 4 worker processes
    with Pool(processes=4) as pool:

        # print "[0, 1, 4,..., 81]"
        print(pool.map(f, range(10)))

        # print same numbers in arbitrary order
        for i in pool.imap_unordered(f, range(10)):
            print(i)

        # evaluate "f(10)" asynchronously
        res = pool.apply_async(f, [10])
        print(res.get(timeout=1))             # prints "100"

        # make worker sleep for 10 secs
        res = pool.apply_async(sleep, [10])
        print(res.get(timeout=1))             # raises multiprocessing.TimeoutError

    # exiting the 'with'-block has stopped the pool

运行此代码后，我得到：

Traceback (most recent call last):
  File "example01.py", line 11, in <module>
    with Pool(processes=4) as pool:
AttributeError: __exit__

不知何故，我发现这是由于 with 关键字。然而，这段代码也在使用with 并且它正在运行：

#!/usr/bin/python
with open("input.csv", "wb") as filePath:
    pass
filePath.close()

当我想运行提到的示例时，我必须按照以下方式对其进行修改：

#!/usr/bin/python

from multiprocessing import Pool
from time import sleep
import traceback

def f(x):
    return x*x

if __name__ == '__main__':
  # start 4 worker processes
  # with Pool(processes=4) as pool:
  try:
    pool = Pool(processes = 4)

    # print "[0, 1, 4,..., 81]"
    print(pool.map(f, range(10)))

    # print same numbers in arbitrary order
    for i in pool.imap_unordered(f, range(10)):
      print(i)

    # evaluate "f(10)" asynchronously
    res = pool.apply_async(f, [10])
    print(res.get(timeout=1))             # prints "100"

    # make worker sleep for 10 secs
    res = pool.apply_async(sleep, [10])
    print(res.get(timeout=1))             # raises multiprocessing.TimeoutError

    # exiting the 'with'-block has stopped the pool

  # http://stackoverflow.com/questions/4990718/python-about-catching-any-exception
  # http://stackoverflow.com/questions/1483429/how-to-print-an-error-in-python
  # http://stackoverflow.com/questions/1369526/what-is-the-python-keyword-with-used-for
  except Exception as e:
    print "Exception happened:"
    print type(e)
    print str(e)
    print traceback.print_exc()

然后输出如下所示：

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
0
1
4
9
16
25
36
49
64
81
100
Exception happened:
<class 'multiprocessing.TimeoutError'>

Traceback (most recent call last):
  File "example01_mod.py", line 29, in <module>
    print(res.get(timeout=1))             # raises multiprocessing.TimeoutError
  File "/usr/lib/python2.7/multiprocessing/pool.py", line 563, in get
    raise TimeoutError
TimeoutError
None

为什么我在使用 with 关键字时会出错，这些代码（带有和 try-catch）是等效的吗？我正在使用 python 2.7.10。

Answer 1

This PEP 描述了使用“with”关键字所需的上下文管理器接口。 Python 2.7 的 Pool 类版本不支持此接口，因此您不能按照您描述的方式使用“with”关键字。

您可以像the example here 那样直接重写代码以不使用并加入/终止池，或者您可以升级到支持池的“with”的 Python 3。

Answer 2

您错误地假设multiprocessor Pool class of Python 2.7 返回context manager。相反，它返回worker processes 的列表。 Pool 已从 Python 3.4.3 转换为上下文管理器对象，如果您的版本早于该版本，则不能将其与上下文管理器一起使用。

要与 with 语句一起使用，表达式应该返回一个带有 enter 和 exit 方法的对象，以便您得到熟悉的错误。

AttributeError: __exit_