[Python-Dev] Backwards incompatible sys.stdout.write() behavior in Python 3 (Was: [Python-ideas] Pythonic buffering in Py3 print())

anatoly techtonik techtonik at gmail.com
Tue Jan 17 11:59:16 CET 2012

On Fri, Jan 13, 2012 at 7:19 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:

> On Fri, 13 Jan 2012 17:00:57 +0100
> Xavier Morel <python-dev at masklinn.net> wrote:
> > FWIW this is not restricted to Linux (the same behavior change can
> > be observed in OSX), and the script is overly complex you can expose
> > the change with 3 lines
> >
> >     import sys
> >     sys.stdout.write('promt>')
> >     sys.stdin.read(1)
> >
> > Python 2 displays "prompt" and terminates execution on [Return],
> > Python 3 does not display anything until [Return] is pressed.
> >
> > Interestingly, the `-u` option is not sufficient to make
> > "prompt>" appear in Python 3, the stream has to be flushed
> > explicitly unless the input is ~16k characters (I guess that's
> > an internal buffer size of some sort)
> "-u" forces line-buffering mode for stdout/stderr, which is already the
> default if they are wired to an interactive device (isattr() returning
> True).
> But this was already rehashed on python-ideas and the bug tracker, and
> apparently Anatoly thought it would be a good idea to post on a third
> medium. Sigh.

If you track this more closely, you'll notice there are four issues
(surprises) from the user point of view:
1. print() buffers output on Python3
2. print() also buffers output on Python2, but only on Linux
3. there is some useless '-u' command line parameter
    (useless, because the last thing user wants is not only care about
Python 2/3, but also how to invoke them)
4. print() is not guilty - it is sys.stdout.write() that buffers output

1-2 discussion was about idea to make new print() function behavior more
'pythonic', i.e. 'user-friendly' or just KISS, which resulted in adding a
flush parameter
3 is a just a side FYI remark
4 doesn't relate to python-ideas anymore about fixing print() - it is about
the *cause* of the problem with print() UX, which is underlying
sys.stdout.write() behavior

I asked 4 here, because it is the more appropriate place not only to ask if
it can be/will be fixed, but also why. The target audience of the question
are developers.

Hope that helps Antoine recover from the sorrow. ;)
anatoly t.
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