Color a Button after subprocess has finished

Question

I have a Tk python program that creates a list of python files in the current directory and generates a button for each of them. When you click a button the corresponding python program is launched via subprocess in a new gnome-terminal. I'd like to switch the button's color to red after the subprocess has finished executing on the new terminal. Unfortunately, the button is changing color almost immediately.

from Tkinter import *
import os, subprocess

root = Tk()
buttonsD = {}

def launch(ourfile):
    p=subprocess.Popen(["gnome-terminal","-e","python " + ourfile], shell=False)
    buttonsD[ourfile].configure(bg='red')

dirlist=os.listdir(os.getcwd())
for fname in dirlist:
    if fname.endswith('py') and fname != 'gui2.py':
        buttonsD[fname] = Button(root,text=fname,command=lambda i=fname: launch(i))
        buttonsD[fname].pack(side=TOP,expand=YES,fill=BOTH)

root.mainloop()

Almost immediately means that I can wait while p.poll == None, and see that it takes a moment for gnome-terminal to be created. But as soon as the terminal is created the button goes red, even though a process is still running in the new terminal. I can't create a new gnome-terminal and then communicate the process I'd like to run either. It seems gnome-terminal just creates a new instance of bash and then returns done, so there's a pipe error if I try to communicate to its stdin.

Answer 1

There are two questions here: what command line to use to launch a Python program in gnome-terminal, and how to use subprocess in a Tkinter app. I only know about the latter.

subprocess.Popen returns immediately, which is why the button is turning red immediately. I think you probably need to make a list of which programs are running. Then write a function poll_processes which calls poll() on each running process, and when the result is not None, removes it from the list and turns the button red.

Then all you have to do is arrange for Tkinter to periodically call that function, which you can do by calling frame.after(msec, poll_processes) to schedule the first call to poll_processes and then having poll_processes do the same thing to schedule the next call.

Answer 2

I believe gnome terminal is doing a double fork, in order to detach itself from the process group of its parent -- so what's actually your subprocess terminates almost immediately, as you observe, and everything is happening in a further descendant that you have no direct access to.

Unfortunately I don't believe gnome terminal offers any way to disable this double fork behavior; so, to find out when the "further descendant" is finished, you'll have to identify that process and monitor it periodically. Interacting directly with it is also a pretty tall order -- no easier than interacting with any "random" process you're not related to:-(.