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

[Guido van Rossum]

I think this may be because in Python 2, there is a coupling between stdin
and stderr (in the C stdlib code) that flushes stdout when you read stdin.
This doesn't seem to be required by the C std, but most implementations
seem to do it.
http://stackoverflow.com/questions/2123528/does-reading-from-stdin-flush-stdout

I think it was a nice feature but I can see problems with it; apps that
want this behavior ought to bite the bullet and flush stdout.

On Fri, Jan 13, 2012 at 7:34 AM, anatoly techtonik <techtonik at gmail.com>wrote:

> Posting to python-dev as it is no more relates to the idea of improving
> print().
>
>
> sys.stdout.write() in Python 3 causes backwards incompatible behavior that
> breaks recipe for unbuffered character reading from stdin on Linux -
> http://code.activestate.com/recipes/134892/  At first I though that the
> problem is in the new print() function, but it appeared that the culprit is
> sys.stdout.write()
>
> Attached is a test script which is a stripped down version of the recipe
> above.
>
> If executed with Python 2, you can see the prompt to press a key (even
> though output on Linux is buffered in Python 2).
> With Python 3, there is not prompt until you press a key.
>
> Is it a bug or intended behavior? What is the cause of this break?
> --
> anatoly t.
>
>
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-- 
--Guido van Rossum (python.org/~guido)
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