subprocess output -> Output widget not working

[Randy Heiland]

I'd like to be able to display output from a program in an Output widget. (And I'd like to be able to kill the program from a widget too). Can someone offer advice?

import ipywidgets as widgets

import subprocess, os

run_output = widgets.Output(layout=widgets.Layout(width='500px', height='100px', border='solid'))

run_output

def run_cb(b):

    #global run_output

    print('run sim...')

    with run_output:

        print('trying subprocess - does hello output show up?')

        args = ['hello']

        run_proc = subprocess.Popen(args)

        #os.system('hello')

    

run_button = widgets.Button(

    description='Run',

    disabled=False,

    button_style='success', # 'success', 'info', 'warning', 'danger' or ''

    tooltip='Run simulation',

)

run_button.on_click(run_cb)

run_button

widgets.VBox([run_button, run_output])

where hello.c is simply:

#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv) {
  int idx;
  for (idx=0; idx<4; idx++) { printf("argc=%d, %d) hello, world...\n",argc,idx); fflush(stdout); sleep(1); }
}

[Thomas Kluyver]

Have a look at this demo notebook:
https://github.com/takluyver/rt2-workshop-jupyter/blob/e7fde6565e28adf31a0f9003094db70c3766bd6d/Subprocess%20output.ipynb

It doesn't put output into a widget, but it's using print(), so it should be easy to combine with a widget.

If you're happy to kill the process by interrupting the kernel from Jupyter, then it's just a matter of catching KeyboardInterrupt in the Python code.

Thomas

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[Randy Heiland]

Thomas,  Thanks very much! Following that example, I am indeed able to capture the output in my Output widget. I'm less clear about how to kill the running job (and keep the rest of the notebook alive). Here's my latest code, fwiw:

import ipywidgets as widgets

from subprocess import Popen, PIPE, STDOUT

run_output = widgets.Output(layout=widgets.Layout(width='500px', height='100px', border='solid'))

def run_cb(b):

    global my_proc

    print('run sim...')

    with run_output:

        args = ['hello']

        my_proc = Popen(args, stdout=PIPE, stderr=STDOUT)

        

    # This should be safe because we're not piping stdin to the process.

    # It gets tricky if we are, because the process can be waiting for input while we're waiting for output.

    while True:

        # Wait for some output, read it and print it.

        with run_output:

            my_output = my_proc.stdout.read1(1024).decode('utf-8')

            print(my_output, end='')

        # Has the subprocess finished yet?

        if my_proc.poll() is not None:

            break

    if my_proc.returncode != 0:

        print("Exited with error code:", my_proc.returncode)

        

def kill_cb(b):

    global my_proc

    print('kill proc...')

    my_proc.terminate()

run_button = widgets.Button(

    description='Run',

    disabled=False,

    button_style='success' # 'success', 'info', 'warning', 'danger' or ''

)

kill_button = widgets.Button(

    description='Kill',

    disabled=False,

    button_style='danger'

)

run_button.on_click(run_cb)

kill_button.on_click(kill_cb)

widgets.VBox([run_button, kill_button, run_output])

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[Thomas Kluyver]

It won't process the message from the kill button until after it's done running the process.

What I'm suggesting is that you rely on Jupyter's interrupt feature (the stop button in the toolbar) to stop the process, instead of making your own kill button. To do that, all you need to do is catch the KeyboardInterrupt exception and call my_proc.terminate() in the exception handler.

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