[Python-Dev] Possible context managers in stdlib

[James Y Knight]

On Jul 8, 2005, at 4:46 PM, Brett Cannon wrote:
> I think having basic context managers in a stdlib module that we know
> for a fact that will be handy is a good idea.  We should keep the list
> short and poignant, but we should have something for people to work
> off of.  The ones I like below for a 'context' module are:
>
>> * builtins: with open/file
>> * sys: with sys.redirected_std[in|out|err]
>> * decimal: with decimal.Context
>> * os: with os.current_directory
>> * mutex: with mutexobj
>> * threading: with threading.Lock
>>              with threading.Condition
>>              with threading.Event
>> * bz2/zipfile/tarfile: with ...open

It is a really bad idea to codify the practice of modifying non- 
threadlocal global state like sys.std[in|out|err] and current  
directory with a context manager. A user can do it to themselves now,  
true, but by putting a context manager for it in the stdlib, you make  
it look like it'd be a local modification when it is not. I can only  
see confusion resulting from this. Users will inevitably try to use  
it like
   with sys.redirected_stderr(f):
     print "hello"
     print "there"
instead of explicitly writing to f with print>> or write(). And that  
is just a terribly bad idea. It looks pretty, yes, but unless  
stdinouterr are made thread-local, it's just asking for trouble.

James