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Date: Mon, 05 Aug 2013 17:08:32 +0100
From: MRAB <python@mrabarnett.plus.com>
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To: python-list@python.org
Subject: Re: Simulate `bash` behaviour using Python and named pipes.
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On 05/08/2013 16:27, Luca Cerone wrote:
> Thanks MRAB,
>>
>> You need to ensure that the pipe is already open at the other end.
>
> So I need to open the process that reads the pipe before writing in
> it?
>
>>
>> Why are you using a named pipe anyway?
>
> For some bug in ipython (see my previous email) I can't use
> subprocess.Popen and pipe in the standard way. One of Ipython
> developers has suggested me to use named pipes as a temporary
> workaround. So I am taking the occasion to learn :)
>
An alternative workaround is to use CPython. :-)

>> If you're talking to another program, then that needs to be
>> running already, waiting for the connection, at the point that you
>> open the named pipe from this end.
>
> I am not entirely sure I got this: ideally I would like to have a
> function that runs an external tool (the equivalent of ls in my
> example) redirecting its output in a named pipe.
>
> A second function (the cat command in my example) would read the
> named_pipe, parse it and extract some features from the output.
>
> I also would like that the named_pipe is deleted when the whole
> communication is ended.
>
>> If you're using a pipe _within_ a program (a queue would be
>> better), then you should opening for writing in one thread and for
>> reading in another.
>
> Let's stick with the pipe :) I will ask about the queue when I
> manage to use pipes ;)
>
> I should have explained better that I have no idea how to run
> threads in Python :): how do I open a thread that executes "ls -lah"
> in background and writes into a named pipe? And how do I open a
> thread that reads from the named pipe?
>
> Can you please post a small example, so that I have something to
> work on?
>
You could try something like this:

os.mkfifo("named_pipe", 0777)
ls_process = subprocess.Popen("ls -lah > named_pipe")
pipe = open("named_pipe", "r")
# Read the output of the subprocess from the pipe.

When the subprocess terminates (look at the docs for Popen objects),
close and delete the fifo.