[New-bugs-announce] [issue16937] -u (unbuffered I/O) command line option documentation mismatch for sys.stdin

[Martijn Pieters]

New submission from Martijn Pieters:

Run `python3.3 -h` and the `-u` option is documented as:

>     -u     : unbuffered binary stdout and stderr; also PYTHONUNBUFFERED=x
>              see man page for details on internal buffering relating to '-u'

Note that only `stdout` and `stderr` are mentioned.

The online documentation at http://docs.python.org/3/using/cmdline.html#cmdoption-u doesn't agree:

> ... the binary layer of the stdin, stdout and stderr streams ...

nor does `man python3.3`, which claims:

>    -u     Force the binary I/O layers of stdin, stdout and stderr to be unbuffered.  The text I/O layer will still
>           be line-buffered.

So, is `stdin` going to be unbuffered, or not, when using `-u`?

Running a simple test shows that `stdin` is firmly being buffered regardless of the `-u` switch:

    $ python3.3 -c 'import sys; print(sys.stdin.buffer, sys.stdout.buffer)'
    <_io.BufferedReader name='<stdin>'> <_io.BufferedWriter name='<stdout>'>
    $ python3.3 -u -c 'import sys; print(sys.stdin.buffer, sys.stdout.buffer)'
    <_io.BufferedReader name='<stdin>'> <_io.FileIO name='<stdout>' mode='wb'>

I'll presume here that `stdin` is deliberately buffered, regardless, and that the documentation and man page are out of date here (in python 2, `-u` does include `stdin`).

----------
assignee: docs at python
components: Documentation
messages: 179718
nosy: docs at python, mjpieters
priority: normal
severity: normal
status: open
title: -u (unbuffered I/O) command line option documentation mismatch for sys.stdin
versions: Python 3.3, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16937>
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