[Python-Dev] sys.__stdout__ - was RE: [Idle-dev] Doc Commentary

[Mark Hammond]

[From a mail in idle-dev - dropped idle-dev and all CCs]

[idle-dev poster]
> > >>> import sys
> > >>> f = open('some-output-file', 'w')
> > >>> sys.stdout = f
> > >>> # do some stuff
> > >>> sys.stdout = sys.__stdout__
> >
> > Because on entry, sys.stdout is already redirected
> > (not reflected in sys.__stdout__)

[Guido]

> Where did you get this idiom?  You should never use sys.__stdout__
> this way!  The proper idiom is
>
>   save_stdout = sys.stdout
>   try:
>       sys.stdout = f
>       # do some stuff
>   finally:
>       sys.stdout = save_stdout
>
> If you know of any places that are promoting direct use of __stdout__,
> please let them know this is wrong.  If you are contemplating to use
> this in code examples you intend to distribute, please don't!

I admit that I first became aware of sys.__stdout__ after seeing this exact
same idiom in other peoples code.  I always assumed that this is _exactly_
what sys.__stdout__ was to be used for!

The following examples exist:

* Tools/unicode/makeunicodedata.py - exactly the idiom you despise.
* Tools/il8n/pygettext.py - ditto
* Tools/idle/PyParse.py - disabled, debugging output function - presumably
to get around sys.stdout redirection - but this sounds suspect as an idiom
to me too!

So what _is_ it intended for?

Mark.