I'm trying to implement an event driven process with system call or subprocess. Basically I want to launch a non-blocking system command and upon completion of that system call, I want a function to be called.
This is so that I can start a GUI progress bar, launch a system command and have the progress bar continue, and when the system call finishes, have the progress bar stop.
What I want to absolutely NOT DO, is to spawn a process, get its process ID and keep checking for the completion of that process in a while loop.
Below is just an example of how I imagine this should work (All of these are inside a 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')
解决方案
To avoid polling subprocess' status, you could use SIGCHLD signal on Unix. To combine it with tkinter's event loop, you could use the self-pipe trick. It also workarounds the possible tkinter + signal issue without the need to wake the event loop periodically.
#!/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')
Output
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