I have an application using curses and threading, that waits for input, then posts it to a queue. The screen very quickly becomes corrupted with odd characters. Initially I tried adding 'with threading.Lock()' to everything that relates to curses to keep the threads from corrupting each other. That didn't work.
In the interest of simplicity, I've distilled the app down to the shortest amount of code I can come up with to show the problem. In the code, I'm adding a string to the screen from the thread, and refreshing. It doesn't take long before errors show up all over the screen. It's probably caused by an escape sequence being written to the screen and being interrupted by another thread mid-write.
Is there a right or smarter way, or a clever trick to getting curses to play nicely with other threads?
#!/usr/bin/python3
# date: 2020.02.29
# platform: raspberry pi 3b+
# python version: 3.5.3
#
# intent: figure out how to get threads to pass messages to the main thread
# without failure. failure: exiting unexpectedly, throwing exceptions, or corrupting the display.
#
# -v0.0: no thread locking; 5 threads; fails almost instantly.
# -v0.1: thread locking every call to curses methods after threads started; still fails.
# -v0.2: reduced # of threads to 1; takes longer to fail.
import sys,os,time,curses,threading
def threadfunc(ch,blocktime,stdscr):
while True:
threadname = 'thread {}'.format(ch)
with threading.Lock():
stdscr.addstr(int(curses.LINES/3)-2,int((curses.COLS - len(threadname))/2),threadname)
stdscr.refresh()
time.sleep(blocktime)
def main(stdscr):
if curses.has_colors() == True:
curses.start_color()
curses.use_default_colors()
curses.init_pair(1,curses.COLOR_GREEN,curses.COLOR_BLUE)
curses.init_pair(2,curses.COLOR_WHITE,curses.COLOR_RED)
stdscr.bkgd(' ',curses.color_pair(1))
curses.curs_set(0) # cursor off.
curses.noecho()
curses.cbreak()
stdscr.keypad(True) # receive special messages.
# instantiate a small window to hold responses to keyboard messages.
xmsg = 32
ymsg = 1
msgwin = curses.newwin(ymsg,xmsg,int(curses.LINES/3),int((curses.COLS - xmsg)/2))
msgwin.bkgd(' ',curses.color_pair(2))
stdscr.noutrefresh()
msgwin.noutrefresh()
curses.doupdate()
# make threads, each with slightly different sleep time:
threadcount = 5
t = []
for i in range(threadcount):
t.append(threading.Thread(target=threadfunc,name='t{}'.format(i),args=(chr(ord('0')+i),0.2+0.02*i,stdscr),daemon=True))
t[i].start()
while True:
with threading.Lock():
key = stdscr.getch() # wait for a character; returns an int; does not raise an exception.
if key == 0x1b: # escape key exits
exitmsg = 'exiting...'
with threading.Lock():
msgwin.erase()
msgwin.addstr(0,int((xmsg-len(exitmsg))/2),exitmsg)
break
else:
feedback = 'received {}'.format(chr(key))
with threading.Lock():
msgwin.erase()
msgwin.addstr(0,int((xmsg-len(feedback))/2),feedback)
with threading.Lock():
msgwin.refresh()
del t # is this the proper way to destroy an object?
exitmsg = 'press any key to exit'
stdscr.addstr(int(curses.LINES/2),int((curses.COLS-len(exitmsg))/2),exitmsg)
stdscr.getkey()
stdscr.keypad(False)
curses.nocbreak()
curses.echo()
curses.endwin()
if __name__ == '__main__':
# Must happen BEFORE calling the wrapper, else escape key has a 1 second delay after pressing:
os.environ.setdefault('ESCDELAY','100') # in mS; default: 1000
curses.wrapper(main)
The trick is to use noutrefresh()/doupdate() instead of refresh() calls. Stage the changes and then handle doupdate()'s in one thread.
#!/usr/bin/python3
# date: 2020.02.29
# platform: raspberry pi 3b+
# python version: 3.5.3
#
# intent: figure out how to get threads to pass messages to the main thread
# without failure. failure: exiting unexpectedly, throwing exceptions, or corrupting the display.
#
# -v0.0: no thread locking; 5 threads; fails almost instantly.
# -v0.1: thread locking every call to curses methods after threads started; still fails.
# -v0.2: reduced # of threads to 1; takes longer to fail.
# -v0.3: no thread locking; using redrawln, redrawwin to fix corruption
# -v0.4: no redrawln; use noutrefresh/doupdate instead of refresh
import sys,os,time,curses,threading
import locale
locale.setlocale(locale.LC_ALL, '')
code = locale.getpreferredencoding()
def threadfunc(ch,blocktime,stdscr):
while True:
threadname = 'thread {}'.format(ch)
stdscr.addstr(int(curses.LINES/3)-2,int((curses.COLS - len(threadname))/2),threadname)
# stdscr.redrawln(int(curses.LINES/3)-2, 1)
stdscr.noutrefresh()
curses.doupdate()
time.sleep(blocktime)
def main(stdscr):
if curses.has_colors() == True:
curses.start_color()
curses.use_default_colors()
curses.init_pair(1,curses.COLOR_GREEN,curses.COLOR_BLUE)
curses.init_pair(2,curses.COLOR_WHITE,curses.COLOR_RED)
stdscr.bkgd(' ',curses.color_pair(1))
curses.curs_set(0) # cursor off.
curses.noecho()
curses.cbreak()
stdscr.keypad(True) # receive special messages.
# instantiate a small window to hold responses to keyboard messages.
xmsg = 32
ymsg = 1
msgwin = curses.newwin(ymsg,xmsg,int(curses.LINES/3),int((curses.COLS - xmsg)/2))
msgwin.bkgd(' ',curses.color_pair(2))
stdscr.noutrefresh()
msgwin.noutrefresh()
curses.doupdate()
# make threads, each with slightly different sleep time:
threadcount = 5
t = []
for i in range(threadcount):
t.append(threading.Thread(target=threadfunc,name='t{}'.format(i),args=(chr(ord('0')+i),0.2+0.02*i,stdscr),daemon=True))
t[i].start()
while True:
key = stdscr.getch() # wait for a character; returns an int; does not raise an exception.
if key == 0x1b: # escape key exits
exitmsg = 'exiting...'
msgwin.erase()
msgwin.addstr(0,int((xmsg-len(exitmsg))/2),exitmsg)
break
else:
feedback = 'received {}'.format(chr(key))
msgwin.erase()
msgwin.addstr(0,int((xmsg-len(feedback))/2),feedback)
msgwin.noutrefresh()
del t # is this the proper way to destroy an object?
exitmsg = 'press any key to exit'
stdscr.addstr(int(curses.LINES/2),int((curses.COLS-len(exitmsg))/2),exitmsg)
stdscr.getkey()
stdscr.keypad(False)
curses.nocbreak()
curses.echo()
curses.endwin()
if __name__ == '__main__':
# Must happen BEFORE calling the wrapper, else escape key has a 1 second delay after pressing:
os.environ.setdefault('ESCDELAY','100') # in mS; default: 1000
curses.wrapper(main)