__main__.py

#!/usr/bin/env python3
"""
- read subprocess output without threads using a socket pair
- show the output in a tkinter GUI (while the process is still running)
- stop subprocess on a button press
"""
import logging
import os
import socket
import sys
import tkinter as tk
from subprocess import Popen, STDOUT


logger = logging.getLogger(__name__)

# define dummy command to generate some output
cmd = [
    sys.executable or "python",
    "-u",
    "-c",
    """
import itertools, time

for i in itertools.count():
    print(i)
    time.sleep(0.1)
""",
]


class ShowProcessOutputDemo:

    def __init__(self, root):
        """Start subprocess, make GUI widgets."""
        self.root = root

        # start subprocess
        self.read_sock, write_sock = socket.socketpair()
        write_sock.set_inheritable(True)
        self.proc = Popen(cmd, stdout=write_sock, stderr=STDOUT)
        logger.info("started")

        # show subprocess' stdout in GUI
        self.read_sock.setblocking(False)
        self.root.createfilehandler(self.read_sock, tk.READABLE, self.read_output)
        self._var = tk.StringVar()  # put subprocess output here
        tk.Label(root, textvariable=self._var).pack()

        # stop subprocess using a button
        tk.Button(root, text="Stop subprocess", command=self.stop).pack()

    def read_output(self, pipe, mask, bufsize=8192):
        """Read subprocess' output, pass it to the GUI."""
        data = os.read(pipe.fileno(), bufsize)
        if not data:  # clean up
            logger.info("eof")
            self.root.deletefilehandler(self.read_sock)
            self.root.after(5000, self.stop)  # stop in 5 seconds
            return
        logger.debug("got: %r", data)
        self._var.set(data.strip(b"\n").decode())

    def stop(self, stopping=[]):
        """Stop subprocess and quit GUI."""
        logger.info("stopping")
        self.proc.terminate()
        self.proc.wait()  # wait for the subprocess' exit
        self.root.destroy()  # exit GUI


logging.basicConfig(level=logging.INFO, format="%(asctime)s %(message)s")
root = tk.Tk()
app = ShowProcessOutputDemo(root)
root.protocol("WM_DELETE_WINDOW", app.stop)  # exit subprocess if GUI is closed
root.mainloop()
logger.info("exited")