[New-bugs-announce] [issue31880] subprocess process interaction with IDLEX GUI causes pygnuplot silent failures

[John Brearley]

New submission from John Brearley <brearley at bell.net>:

There is an interesting interaction between the IDLEX GUI and subprocess module that causes pygnuplot silent failures. The example.py script below works fine when run from the WinPython Command Prompt.exe terminal window. The script will popup a new window from gnuplot with the desired graph, and in the same directory save the generated raw data as example.out and the generated graph as example.pdf. If you erase the generated files and then run the same script via the IDLEX GUI, there is no graph window created and only the raw data example.out file is created. There are no error messages.

The PyGnuplot module sets up a subprocess.Popen pipe as persistant connection to the gnuplot.exe utility. This allows the top level script to send multiple commands to gnuplot to compose the desired graph, and then finally save the file to disk. This all works fine when run from the WinPython Command Prompt.exe terminal window. However something subtle is breaking when the same script is run from the IDLEX GUI environment. It is not at all obvious if the subprocess module is not as forgiving as it needs to be of the IDLEX GUI input or if the IDLEX GUI is breaking the rules somewhere. I will start by asking the subprocess module team to comment.

I did try adding some trace code to the PyGnuplot.py module. In particular I turned on stdout=subprocess.PIPE  and stderr=subprocess.PIPE. I did proc.poll() before/after the command is sent to gnuplot. Interestingly, when I did proc.communicate(timeout=0) to do an immediate read for any stdout/err data, this seems to cause the
 subsequent write to the pipe.stdin to fail, saying the file is already closed. In another learning exercise script for subprocess, the communicate() method does not seem to interfere with the pipe behavior.

This issue is quite repeatable on demand. I set up a second PC and reproduced the issue on demand on the second PC. I am using WinPython 3.6.1 on Windows 7 with gnuplot 5.2.0 on both PC.

Here are the links to the various components needed to setup the environment to reproduce this issue:
1) gnuplot 5.2.0   https://sourceforge.net/projects/gnuplot/files/gnuplot/5.2.0/
2) pygnuplot 0.10.0  https://pypi.python.org/pypi/PyGnuplot/0.10.0
3) There is a one-line fix to PyGnuplot.py needed   https://github.com/benschneider/PyGnuplot/blob/master/PyGnuplot.py
4) example.py script  https://github.com/benschneider/PyGnuplot/blob/master/example.py

WinPython 3.6.1 installer comes with subprocess.py as part of the package, no version info that I can find.

When installing gnuplot there is an advanced option to check on that will automatically update your PATH variable. If you dont do this, then you must manully add "C:\gnuplot\bin" (or whatever the directory is)
 to your PATH variable. To check if gnuplot is correctly installed, from a DOS prompt terminal window, type "gnuplot -V". You should get a one line response "gnuplot 5.2 patchlevel 0".

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components: Interpreter Core, Windows
messages: 305073
nosy: jbrearley, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: subprocess process interaction with IDLEX GUI causes pygnuplot silent failures
type: behavior
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31880>
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