Piping python's output

Dave Farrance

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Sep 19, 2012, 7:02:37 AM9/19/12

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I've just been tracing a bug in a script I'd written which included the
Ubuntu "add-apt-repository" command, and it didn't output its prompt text
until *after* its operation had completed. Something to do with it being
a Python script, and me piping its output for logging.

After trying some python test-cases, I'd guess it's down to this... here's
a python one-liner that prompts for the Enter-key then says OK:

$ python -c 'import os; raw_input("Hit Enter"); os.system("echo OK")'

So far so good, but put a pipe on the output:

$ python -c 'import os; raw_input("Hit Enter"); os.system("echo OK")' |tee

And now the prompt doesn't appear until *after*. So I guess I'd better
forget about piping the output of certain Ubuntu commands for logging. Or
just get used to typing blind. Or am I missing something?

Giovanna Stefani

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Sep 19, 2012, 7:39:09 AM9/19/12

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Dave Farrance scrive:

> And now the prompt doesn't appear until *after*. So I guess I'd better
> forget about piping the output of certain Ubuntu commands for logging.
> Or just get used to typing blind. Or am I missing something?

Add a logging command at the end in place of the pipe.

--
Giovanna
Debian 6 user

Tony Houghton

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Sep 19, 2012, 9:07:38 AM9/19/12

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In <gl4j58to67ibnctqt...@4ax.com>,

How about this?:

import sys; print "Hit Enter",; sys.stdout.flush(); raw_input(""); print "OK"

(although using print is a bit awkward these days with the transition
from python 2 to 3).

--
TH * http://www.realh.co.uk

Dave Farrance

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Sep 19, 2012, 12:18:10 PM9/19/12

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Tony Houghton <h...@realh.co.uk> wrote:

>How about this?:
>
>import sys; print "Hit Enter",; sys.stdout.flush(); raw_input(""); print "OK"
>
>(although using print is a bit awkward these days with the transition
>from python 2 to 3).

OK, thanks. I didn't want to hack the "add-apt-repository" Ubuntu tool
itself, but it turned out to be easy to spot the text-output lines
describing what the tool was about to do followed by the user-prompt, so I
inserted "sys.stdout.flush()" there, and that works.

However, hacking stuff in /usr/bin is storing up trouble, so I'd rather
find a way to call the tool as-is with my bash script and still be able to
log its output. I was wondering why python is happy to output text to
stdout unless it's to a pipe making this flush command necessary. How
does it "know" that the pipe is there?

Dave Farrance

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Sep 19, 2012, 1:21:33 PM9/19/12

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Dave Farrance <DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> wrote:

> ... I was wondering why python is happy to output text to

>stdout unless it's to a pipe making this flush command necessary. How
>does it "know" that the pipe is there?

Never mind. More Googling gave me the answer. It's because bash buffers
stdout in various ways. Prefixing a command with "stdbuf -oL" switches
the buffering to line-mode, and that's the solution in this case.

Graham Murray

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Sep 19, 2012, 4:27:44 PM9/19/12

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Dave Farrance <DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> writes:

> However, hacking stuff in /usr/bin is storing up trouble, so I'd rather
> find a way to call the tool as-is with my bash script and still be able to
> log its output. I was wondering why python is happy to output text to
> stdout unless it's to a pipe making this flush command necessary. How
> does it "know" that the pipe is there?

It does not actually know it is a pipe. What is does know is that it is
not a terminal - isatty(3) tells it that the file descriptor is not a
terminal.