[Python-ideas] Replacing the standard IO streams (was Re: changing sys.stdout encoding)

Paul Moore p.f.moore at gmail.com
Sat Jun 9 13:00:37 CEST 2012

On 9 June 2012 10:55, Nick Coghlan <ncoghlan at gmail.com> wrote:
> So, after much digging, it appears the *right* way to replace a
> standard stream in Python 3 after application start is to do the
> following:
>    sys.stdin = open(sys.stdin.fileno(), 'r', <new settings>)
>    sys.stdout = open(sys.stdout.fileno(), 'w', <new settings>)
>    sys.stderr = open(sys.stderr.fileno(), 'w', <new settings>)
> Ditto for the other standard streams. It seems it already *is* as
> simple as with any other file, we just collectively forgot about:

One minor point - if sys.stdout is redirected, *and* you have already
written to sys.stdout, this resets the file pointer. With test.py as

import sys
sys.stdout = open(sys.stdout.fileno(), 'w', encoding='utf-8')

test.py >a gives one line in a, not two (tested on Windows, Unix may
be different). And changing to "a" doesn't resolve this...

Of course, the actual use case is to change the encoding before
anything is written - so maybe a small note saying "don't do this" is
enough. But it's worth mentioning before we get the bug report saying
"Python lost my data" :-)


More information about the Python-ideas mailing list