There are a number of issues with subprocess and __exit__ , 12494 status fixed among them.

Program (which doesn't work as I had hoped, but that's not the issue):


'file p.py'

import subprocess as S

with S.Popen(('cat','-n',),shell=False,stdin=S.PIPE,stdout=S.PIPE,bufsize=1,universal_newlines=True) as p:
    p.stdin.write('these\n')
    print(p.stdout.readline())




Use:

$ python3 p.py

Traceback (most recent call last):
  File "p.py", line 5, in <module>
    print(p.stdout.readline())
KeyboardInterrupt
$
$ python p.py
Traceback (most recent call last):
  File "p.py", line 3, in <module>
    with S.Popen(('cat','-n',),shell=False,stdin=S.PIPE,stdout=S.PIPE,bufsize=1,universal_newlines=True) as p:
AttributeError: __exit__
$
$ python
Python 2.7.1+ (r271:86832, Apr 11 2011, 18:13:53) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Context management support was added in 3.2.  It is not supported in 2.7.

To clarify: Python 2.5 and higher support the with statement syntax, but not all classes that could benefit from it have the __enter__ and __exit__ methods.  So you don’t get a SyntaxError in Python 2.7 when you write “with Popen(...)”, but you get an AttributeError because Popen in 2.7 does not do its part to support context management.  The built-in open function, on the contrary, supports with in 2.5 and higher.