reflection of the overhead incurred for switching the interpreter (and CPU) between
multiple threads—but again, this can vary.

Example 11-10. PP4E\Gui\MovingPics\movingpics_threads.py

"""
PyDraw-threads: use threads to move objects; works okay on Windows
provided that canvas.update() not called by threads (else exits with
fatal errors, some objs start moving immediately after drawn, etc.);
at least some canvas method calls must be thread safe in tkinter;
this is less smooth than time.sleep, and is dangerous in general:
threads are best coded to update global vars, not change GUI;
"""

import _thread as thread, time, sys, random
from tkinter import Tk, mainloop
from movingpics import MovingPics, pickUnits, pickDelays

class MovingPicsThreaded(MovingPics):
def init(self, parent=None):
MovingPics.init(self, parent)
self.mutex = thread.allocate_lock()
import sys
#sys.setcheckinterval(0) # switch after each vm op: doesn't help

def onMove(self, event):
object = self.object
if object and object not in self.moving:
msecs = int(pickDelays[0] * 1000)
parms = 'Delay=%d msec, Units=%d' % (msecs, pickUnits[0])
self.setTextInfo(parms)
#self.mutex.acquire()
self.moving.append(object)
#self.mutex.release()
thread.start_new_thread(self.doMove, (object, event))

def doMove(self, object, event):
canvas = event.widget
incrX, reptX, incrY, reptY = self.plotMoves(event)
for i in range(reptX):
canvas.move(object, incrX, 0)

canvas.update()

time.sleep(pickDelays[0]) # this can change
for i in range(reptY):
canvas.move(object, 0, incrY)

canvas.update() # update runs other ops

time.sleep(pickDelays[0]) # sleep until next move
#self.mutex.acquire()
self.moving.remove(object)
if self.object == object: self.where = event
#self.mutex.release()

if name == 'main':
root = Tk()

746 | Chapter 11: Complete GUI Programs