Issue 16937: -u (unbuffered I/O) command line option documentation mismatch for sys.stdin (original) (raw)

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I'll presume here that stdin is deliberately buffered, regardless, and that the documentation and man page are out of date here (in python 2, -u does include stdin).