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Why can't I restore my redirected stdout?

David Bolen db3l at fitlinxx.com
Thu Jun 8 18:31:03 EDT 2000 
"Raymond Ng Tong Leng" <think2 at pd.jaring.my> writes:

> Remco Gerlich wrote in message ...
> >
> >I just typed this in, and it worked fine! Something else must be happening.
> >Are you in idle or so? Does it work inside a normal python interpreter?
> >What's your platform?
> >
> 
> Yes, I was using IDLE 0.4 with Python 1.5.2 on Win NT. When I tried it
> inside a normal Python interpreter like you suggested, it worked fine. I
> hope Guido fixed this bug... sorry, improved on this feature :) in the
> latest version of IDLE.

What's happening in IDLE is that sys.stdout is already being re-routed
(so it can be displayed in the window after being colored and such),
but sys.__stdout__ is still the true original stdout.  For example,
the following is from an IDLE window:

    >>> import sys
    >>> print sys.stdout
    <PyShell.PseudoFile instance at 85d5a0>
    >>> print sys.__stdout__
    <open file '<stdout>', mode 'w' at 792980>

And in the case of IDLE, it's running as a Windows application that
doesn't actually have a console, so the true <stdout> file doesn't go
anywhere.  Thus when you try to "restore" to sys.__stdout__ you aren't
really restoring the initial IDLE output file, but a pointer to a
console that technically doesn't exist (at least not with a window).

I guess by definition, this is probably the "right" thing to do, since
sys.__stdout__ is supposed to be the true original stdout when the
Python application was started, and even IDLE is just a Python
application.  So it probably depends on your view of how closely IDLE
should emulate running a textual Python in a real console.

As one workaround, if you want your original approach (restoring to
sys.__stdout__) to work, you could try re-assigning sys.__stdout__ when
you start up your IDLE session, e.g.:

    >>> sys.__stdout__ = sys.stdout

which works in a quick test, but I noticed that the PyShell class in
IDLE uses sys.__stdout__ for its own restoration, so I don't know if
there might be a later gotcha.

Where IDLE redirects sys.stdout is in PyShell.py, so you could tweak
that to do whatever you want if you really wanted different behavior.
Note that you can see other places in IDLE where sys.stdout is
temporarily redirected and then restored.

What's probably a better, more general approach, is to just save
sys.stdout yourself and restore it later.  If you're building a module
that may be imported from other modules that's actually the safer
approach anyway, since in that case you'd want to restore the output
to what sys.stdout was before you re-routed it, which may or may not
be the same as the very original sys.stdout.

--
-- David
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