Python event management system

Question

I am trying to implement an event driven process with a system call or subprocess. Basically I want to run a non-blocking system command, and after completing this system call, I want to call a function. This means that I can start the GUI progress bar, run the system command and continue the progress bar, and when the system call ends, stop the progress bar.

What I want absolutely NOT to do is create a process, get its process ID and continue checking for the completion of this process in the while loop.

Below is an example of how I think this should work (they are all inside the class)

def launchTool(self): self.progressbar.config(mode = 'indeterminate') self.progressbar.start(20) self.launchButton.config(state = 'disabled') self.configCombobox.config(state = 'disabled') ## here the "onCompletion" is a pointer to a function call("/usr/bin/make psf2_dcf", shell=True, onCompletion = self.toolCompleted) def onCompletion(self): print('DONE running Tool') self.progressbar.stop() self.launchButton.config(state = 'normal') self.configCombobox.config(state = 'normal') 

Answer 1

To avoid polling subprocess status, you can use the SIGCHLD signal on Unix. To combine it with the tkinter event loop, you can use the trick with your own pipes . It also bypasses solutions to a possible problem with the tkinter + signal , without requiring periodic wake- ups of the event loop.

 #!/usr/bin/env python3 import logging import os import signal import subprocess import tkinter info = logging.getLogger(__name__).info def on_signal(pipe, mask, count=[0]): try: signals = os.read(pipe, 512) except BlockingIOError: return # signals have been already dealt with # from asyncio/unix_events.py #+start # Because of signal coalescing, we must keep calling waitpid() as # long as we're able to reap a child. while True: try: pid, status = os.waitpid(-1, os.WNOHANG) except ChildProcessError: info('No more child processes exist.') return else: if pid == 0: info('A child process is still alive. signals=%r%s', signals, ' SIGCHLD'*(any(signum == signal.SIGCHLD for signum in signals))) return #+end # you could call your callback here info('{pid} child exited with status {status}'.format(**vars())) count[0] += 1 if count[0] == 2: root.destroy() # exit GUI logging.basicConfig(format="%(asctime)-15s %(message)s", datefmt='%F %T', level=logging.INFO) root = tkinter.Tk() root.withdraw() # hide GUI r, w = os.pipe2(os.O_NONBLOCK | os.O_CLOEXEC) signal.set_wakeup_fd(w) root.createfilehandler(r, tkinter.READABLE, on_signal) signal.signal(signal.SIGCHLD, lambda signum, frame: None) # enable SIGCHLD signal.siginterrupt(signal.SIGCHLD, False) # restart interrupted syscalls automatically info('run children') p = subprocess.Popen('sleep 4', shell=True) subprocess.Popen('sleep 1', shell=True) root.after(2000, p.send_signal, signal.SIGSTOP) # show that SIGCHLD may be delivered root.after(3000, p.send_signal, signal.SIGCONT) # while the child is still alive root.after(5000, lambda: p.poll() is None and p.kill()) # kill it root.mainloop() info('done') 

Exit

 2015-05-20 23:39:50 run children 2015-05-20 23:39:51 16991 child exited with status 0 2015-05-20 23:39:51 A child process is still alive. signals=b'\x11' SIGCHLD 2015-05-20 23:39:52 A child process is still alive. signals=b'\x11' SIGCHLD 2015-05-20 23:39:53 A child process is still alive. signals=b'\x11' SIGCHLD 2015-05-20 23:39:54 16989 child exited with status 0 2015-05-20 23:39:54 No more child processes exist. 2015-05-20 23:39:54 done