Problems related to interception of a subprocess in real time
I spent about 6 hours on stack overflow, rewrote my code in Python and tried to get it to work. It just isn't true. It doesn't matter what I do.
Purpose: Get the live output of a subprocess in a tkinter textbox.
Problem: I can't figure out how to get Popen to work in real time. It seems to hang until the process completes. (Running on its own, the process works completely as expected, so it's only this thing that has a bug)
Relevant code:
import os
import tkinter
import tkinter.ttk as tk
import subprocess
class Application (tk.Frame):
process = 0
def __init__ (self, master=None):
tk.Frame.__init__(self, master)
self.grid()
self.createWidgets()
def createWidgets (self):
self.quitButton = tk.Button(self, text='Quit', command=self.quit)
self.quitButton.grid()
self.console = tkinter.Text(self)
self.console.config(state=tkinter.DISABLED)
self.console.grid()
def startProcess (self):
dir = "C:/folder/"
self.process = subprocess.Popen([ "python", "-u", dir + "start.py" ], stdout=subprocess.PIPE, stdin=subprocess.PIPE, stderr=subprocess.PIPE, cwd=dir)
self.updateLines()
def updateLines (self):
self.console.config(state=tkinter.NORMAL)
while True:
line = self.process.stdout.readline().decode().rstrip()
if line == '' and self.process.poll() != None:
break
else:
self.console.insert(tkinter.END, line + "\n")
self.console.config(state=tkinter.DISABLED)
self.after(1, self.updateLines)
app = Application()
app.startProcess()
app.mainloop()
Also, feel free to destroy my code if it's poorly written. This is my first python project, I don't expect to be good on it yet.
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The problem is that process.stdout.readline()
will block until the full row is available. This means that the condition line == ''
will never be met until the process ends. You have two options.
First, you can set stdout to non-blocking and manage the buffer yourself. It will look something like this. EDIT: As Terry Ian Reedy said, this is a Unix solution. The second option 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 option is to have a separate thread read lines into the queue. Then pop-ups appear from the queue. It will look something like this.
from threading import Thread
from queue import Queue, Empty
def 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)
Stream tracing is probably safer. I'm not sure if setting stdout to non-blocking will work on all platforms.
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