Python tkinter after

Question

I use tinter.after() for refreshing the display of an analog clock on a raspberry pi every 200ms. At the beginning it's OK but gradually the time between each refresh reaches about 2-3 seconds. Is there any solution to keep the refresh interval to 200ms?

#!/usr/bin/env python
# coding: UTF-8
# license: GPL
#
## @package _08c_clock
#
#  A very simple analog clock.
#
#  The program transforms worldcoordinates into screencoordinates 
#  and vice versa according to an algorithm found in: 
#  "Programming principles in computer graphics" by Leendert Ammeraal.
#
#  Based on the code of Anton Vredegoor (anton.vredegoor@gmail.com) 
#
#  @author Paulo Roma
#  @since 01/05/2014
#  @see https://code.activestate.com/recipes/578875-analog-clock
#  @see http://orion.lcg.ufrj.br/python/figuras/fluminense.png

import sys, types, os
from time import localtime
from datetime import timedelta,datetime
from math import sin, cos, pi
from threading import Thread
try:
    from tkinter import *       # python 3
except ImportError:
    try:
       from mtTkinter import *  # for thread safe
    except ImportError:
       from Tkinter import *    # python 2

hasPIL = True
# we need PIL for resizing the background image
# in Fedora do: yum install python-pillow-tk
# or yum install python3-pillow-tk
try:
    from PIL import Image, ImageTk
except ImportError:
    hasPIL = False

## Class for handling the mapping from window coordinates
#  to viewport coordinates.
#
class mapper:
    ## Constructor.
    #
    #  @param world window rectangle.
    #  @param viewport screen rectangle.
    #
    def __init__(self, world, viewport):
        self.world = world 
        self.viewport = viewport
        x_min, y_min, x_max, y_max = self.world
        X_min, Y_min, X_max, Y_max = self.viewport
        f_x = float(X_max-X_min) / float(x_max-x_min) 
        f_y = float(Y_max-Y_min) / float(y_max-y_min) 
        self.f = min(f_x,f_y)
        x_c = 0.5 * (x_min + x_max)
        y_c = 0.5 * (y_min + y_max)
        X_c = 0.5 * (X_min + X_max)
        Y_c = 0.5 * (Y_min + Y_max)
        self.c_1 = X_c - self.f * x_c
        self.c_2 = Y_c - self.f * y_c

    ## Maps a single point from world coordinates to viewport (screen) coordinates.
    #
    #  @param x, y given point.
    #  @return a new point in screen coordinates.
    #
    def __windowToViewport(self, x, y):
        X = self.f *  x + self.c_1
        Y = self.f * -y + self.c_2      # Y axis is upside down 
        return X , Y

    ## Maps two points from world coordinates to viewport (screen) coordinates.
    #
    #  @param x1, y1 first point.
    #  @param x2, y2 second point.
    #  @return two new points in screen coordinates.
    #
    def windowToViewport(self,x1,y1,x2,y2):
        return self.__windowToViewport(x1,y1),self.__windowToViewport(x2,y2)

## Class for creating a new thread.
#
class makeThread (Thread):
      """Creates a thread."""

      ## Constructor.
      #  @param func function to run on this thread.
      #
      def __init__ (self,func):
          Thread.__init__(self)
          self.__action = func
          self.debug = False

      ## Destructor.
      #
      def __del__ (self):
          if ( self.debug ): print ("Thread end")

      ## Starts this thread.
      #
      def run (self):
          if ( self.debug ): print ("Thread begin")
          self.__action()

## Class for drawing a simple analog clock.
#  The backgroung image may be changed by pressing key 'i'.
#  The image path is hardcoded. It should be available in directory 'images'.
#
class clock:
    ## Constructor.
    #
    #  @param deltahours time zone.
    #  @param sImage whether to use a background image.
    #  @param w canvas width.
    #  @param h canvas height.
    #  @param useThread whether to use a separate thread for running the clock.
    #
    def __init__(self,root,deltahours = 0,sImage = True,w = 400,h = 400,useThread = False):
        self.world       = [-1,-1,1,1]
        self.imgPath     = './images/fluminense.png'  # image path
        if hasPIL and os.path.exists (self.imgPath):
           self.showImage = sImage
        else:
           self.showImage = False

        self.setColors()
        self.circlesize  = 0.09
        self._ALL        = 'handles'
        self.root        = root
        width, height    = w, h
        self.pad         = width/16

        if self.showImage:
           self.fluImg = Image.open(self.imgPath)

        self.root.bind("<Escape>", lambda _ : root.destroy())
        self.delta = timedelta(hours = deltahours)  
        self.canvas = Canvas(root, width = width, height = height, background = self.bgcolor)
        viewport = (self.pad,self.pad,width-self.pad,height-self.pad)
        self.T = mapper(self.world,viewport)
        self.root.title('Clock')
        self.canvas.bind("<Configure>",self.resize)
        self.root.bind("<KeyPress-i>", self.toggleImage)
        self.canvas.pack(fill=BOTH, expand=YES)

        if useThread:
           st=makeThread(self.poll)
           st.debug = True
           st.start()
        else:
           self.poll()

    ## Called when the window changes, by means of a user input.
    #
    def resize(self,event):
        sc = self.canvas
        sc.delete(ALL)            # erase the whole canvas
        width  = sc.winfo_width()
        height = sc.winfo_height()

        imgSize = min(width, height)
        self.pad = imgSize/16
        viewport = (self.pad,self.pad,width-self.pad,height-self.pad)
        self.T = mapper(self.world,viewport)

        if self.showImage:
           flu = self.fluImg.resize((int(0.8*0.8*imgSize), int(0.8*imgSize)), Image.ANTIALIAS) 
           self.flu = ImageTk.PhotoImage(flu)
           sc.create_image(width/2,height/2,image=self.flu)
        else:
           self.canvas.create_rectangle([[0,0],[width,height]], fill = self.bgcolor)

        self.redraw()             # redraw the clock    

    ## Sets the clock colors.
    #
    def setColors(self):
        if self.showImage:
           self.bgcolor     = 'antique white'
           self.timecolor   = 'dark orange'
           self.circlecolor = 'dark green'
        else:
           self.bgcolor     = '#000000'
           self.timecolor   = '#ffffff'
           self.circlecolor = '#808080'

    ## Toggles the displaying of a background image.
    #
    def toggleImage(self,event):
        if hasPIL and os.path.exists (self.imgPath):
           self.showImage = not self.showImage
           self.setColors()
           self.resize(event)

    ## Redraws the whole clock.
    # 
    def redraw(self):
        start = pi/2              # 12h is at pi/2
        step = pi/6
        for i in range(12):       # draw the minute ticks as circles
            angle =  start-i*step
            x, y = cos(angle),sin(angle)
            self.paintcircle(x,y)
        self.painthms()           # draw the handles
        if not self.showImage:
           self.paintcircle(0,0)  # draw a circle at the centre of the clock

    ## Draws the handles.
    # 
    def painthms(self):
        self.canvas.delete(self._ALL)  # delete the handles
        T = datetime.timetuple(datetime.utcnow()-self.delta)
        x,x,x,h,m,s,x,x,x = T
        self.root.title('%02i:%02i:%02i' %(h,m,s))
        angle = pi/2 - pi/6 * (h + m/60.0)
        x, y = cos(angle)*0.70,sin(angle)*0.70   
        scl = self.canvas.create_line
        # draw the hour handle
        scl(self.T.windowToViewport(0,0,x,y), fill = self.timecolor, tag=self._ALL, width = self.pad/3)
        angle = pi/2 - pi/30 * (m + s/60.0)
        x, y = cos(angle)*0.90,sin(angle)*0.90
        # draw the minute handle
        scl(self.T.windowToViewport(0,0,x,y), fill = self.timecolor, tag=self._ALL, width = self.pad/5)
        angle = pi/2 - pi/30 * s
        x, y = cos(angle)*0.95,sin(angle)*0.95   
        # draw the second handle
        scl(self.T.windowToViewport(0,0,x,y), fill = self.timecolor, tag=self._ALL, arrow = 'last')

    ## Draws a circle at a given point.
    # 
    #  @param x,y given point.
    # 
    def paintcircle(self,x,y):
        ss = self.circlesize / 2.0
        sco = self.canvas.create_oval
        sco(self.T.windowToViewport(-ss+x,-ss+y,ss+x,ss+y), fill = self.circlecolor)

    ## Animates the clock, by redrawing everything after a certain time interval. 
    #
    def poll(self):
        self.redraw()
        self.root.after(200,self.poll)

## Main program for testing.
#
#  @param argv time zone, image background flag,
#         clock width, clock height, create thread flag.
#
def main(argv=None):
    if argv is None:
       argv = sys.argv
    if len(argv) > 2:
       try:
           deltahours = int(argv[1])
           sImage = (argv[2] == 'True')
           w = int(argv[3])
           h = int(argv[4])
           t = (argv[5] == 'True')
       except ValueError:
           print ("A timezone is expected.")
           return 1
    else:
       deltahours = 3
       sImage = True  
       w = h = 400
       t = False

    root = Tk()
    root.geometry ('+0+0')
    # deltahours: how far are you from utc?
    # Sometimes the clock may be run from another timezone ...
    clock(root,deltahours,sImage,w,h,t)

    root.mainloop()

if __name__=='__main__':
    sys.exit(main())

Answer 1

No, there is not. Tkinter only guarantees that the event will be triggered sometime after the delay. Usually the difference is only a millisecond or two, but it can be longer.

That being said, if you trigger the new event before doing any work, you'll likely witness less drift than if you do it after doing work.

For example, consider this code:

def callback(self):
    <do some work>
    self.after(200, self.callback)

This will drift by whatever amount of time it takes to "do some work". If that work takes 100ms, than the next call won't happen until 300ms after the current call.

If you want as little drift as possible you can schedule the new callback immediately, before any other code:

def callback(self):
    self.after(200, self.callback)
    <do some work>

If you want to do something every 200 ms, choose the second method. If you want to wait for 200ms after the first callback has finished, use the first method.

Answer 2

You can get closer timing with something like this:

def callback(self):
    time_start = time.time()
    # DO YOUR WORK HERE
    time_used = (time.time() - time_start) * 1000
    sleep = 200 - time_used
    if sleep < 1:
        sleep = 1
    self.after(sleep, self.callback)

I've been using this successfully to ensure puslseaudio sets volume every 1/10th second while python calculates the next volume and factors in time for subprocess Linux shell command launched in background to complete.