Issues intercepting subprocess output in real time

The problem here is that process.stdout.readline() will block until a full line is available. This means the condition line == '' will never be met until the process exits. You have two options around this.

First you can set stdout to non-blocking and manage a buffer yourself. It would look something like this. EDIT: As Terry Jan Reedy pointed out this is a Unix only solution. The second alternative should be preferred.

import fcntl...    def startProcess(self):        self.process = subprocess.Popen(['./subtest.sh'],            stdout=subprocess.PIPE,            stdin=subprocess.PIPE,            stderr=subprocess.PIPE,            bufsize=0) # prevent any unnecessary buffering        # set stdout to non-blocking        fd = self.process.stdout.fileno()        fl = fcntl.fcntl(fd, fcntl.F_GETFL)        fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)        # schedule updatelines        self.after(100, self.updateLines)    def updateLines(self):        # read stdout as much as we can        line = ''        while True:            buff = self.process.stdout.read(1024)            if buff:                buff += line.decode()            else:                break        self.console.config(state=tkinter.NORMAL)        self.console.insert(tkinter.END, line)        self.console.config(state=tkinter.DISABLED)        # schedule callback        if self.process.poll() is None:            self.after(100, self.updateLines)

The second alternative is to have a separate thread read the lines into a queue. Then have updatelines pop from the queue. It would look something like this

from threading import Threadfrom queue import Queue, Emptydef readlines(process, queue):    while process.poll() is None:        queue.put(process.stdout.readline())...    def startProcess(self):        self.process = subprocess.Popen(['./subtest.sh'],            stdout=subprocess.PIPE,            stdin=subprocess.PIPE,            stderr=subprocess.PIPE)        self.queue = Queue()        self.thread = Thread(target=readlines, args=(self.process, self.queue))        self.thread.start()        self.after(100, self.updateLines)    def updateLines(self):        try:            line = self.queue.get(False) # False for non-blocking, raises Empty if empty            self.console.config(state=tkinter.NORMAL)            self.console.insert(tkinter.END, line)            self.console.config(state=tkinter.DISABLED)        except Empty:            pass        if self.process.poll() is None:            self.after(100, self.updateLines)

The threading route is probably safer. I'm not positive that setting stdout to non-blocking will work on all platforms.