关于python：关键字在打开文件时有效，但在调用函数时无效

with keyword works when openning file but not when calling a function

我正在试验多处理模块，并从这个页面复制示例代码。下面是一个例子：

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#!/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：

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

不知怎么的，我发现这是由于with关键字造成的。但是，此代码也在使用with，它正在运行：

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

当我想运行上面提到的示例时，我必须按照以下方式修改它：

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#!/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()

输出结果如下：

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[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(with和try catch)是否等价？我使用的是python 2.7.10。