stderr writting before stdout

[Souvik Dutta]

Thank you I understood. Sorry for top posting... 😛


On Sun, 24 May, 2020, 11:34 am Cameron Simpson, <cs at cskk.id.au> wrote:

> Please don't top post; I have rearranged your message so that the
> discussion reads from top to bottom. Reponse below.
>
> On 24May2020 10:04, Souvik Dutta <souvik.viksou at gmail.com> wrote:
> >On Sun, 24 May, 2020, 9:57 am Souvik Dutta, <souvik.viksou at gmail.com>
> >wrote:
> >> Is there any precedence or priority order by which sys.stderr.write()
> >> and
> >> sys.stdout.write() works. Because when running the below code...
> >>
> >> import sys
> >> sys.stdout.write("Writting")
> >> sys.stderr.write("No errors \n")
> >>
> >> No errors is written (displayed) first and writting is written later.
> Why
> >> does this happen?
> >
> >Also this code maintains order i.e. writting is displayed before no
> >errors.
> >Why is that?
> >
> >import sys
> >sys.stdout.write("Writting \n")
> >sys.stderr.write("No errors \n")
>
> This is all to do with buffering.
>
> Output streams are buffered (unless you go out of your way to avoid it).
> This means that for throughput performance reasons, when you go
> f.write(...) the daa are written into an in memory buffer, and not yet
> to the file itself. The buffer is conventionally of fixe size.
>
> A fully buffered stream tries to minimise the OS-write calls (which copy
> from the pogramme to the operating system) because they are
> comparitively expensive. For thorughtput, writing a lot of data in bulk,
> this is a win because the OS-write calls are as few as possibly.
>
> An unbuffered stream calls the OS-write as soon as any data are
> received; there wouldn't even be an in memory beffer at all. This
> presents data to the OS as soon as possible, at the cost of more frequnt
> OS-write calls.
>
> In between these 2 extremes are onther policies. One such is line
> buffering: the contents of the buffer are written to the OS when there
> is a complete line in the bufeer (i.e. when a newline character arrives
> in the data). This is common on terminals, and writes complete lines to
> the terminal. This is common for writing "text" to a terminal, and makes
> more OS-writes that full buffeing and fewer than unbuffered as a
> comprimise to present timely display to a human.
>
> You can force an OS-write of a buffered stream by calling its flush()
> method.
>
> Conventionally, on UNIX systems, an output stream connected to a regular
> file is fully buffered, to maximise bulk data writing throughput. An
> output stream connected to a terminal is line buffered. Except for
> stderr, which is _unbuffered_ because it is felt that error messages
> should always show up as soon as possible.
>
> These different policies explain the behaviour you see.
>
> By inserting:
>
>     sys.stdout.flush()
>
> calls at various points for can force OS-write calls and see data
> immediately, which will make this more clear to you.
>
> Cheers,
> Cameron Simpson <cs at cskk.id.au>
> --
> https://mail.python.org/mailman/listinfo/python-list
>