Tkinter understanding mainloop

Question

Till now  I used to end my Tkinter programs with  tk mainloop    or nothing would show up  See example  from Tkinter import   import random import time  tk   Tk   tk title    quot Game quot  tk resizable 0 0  tk wm attributes  quot -topmost quot   1   canvas   Canvas tk  width 500  height 400  bd 0  highlightthickness 0  canvas pack    class Ball      def   init   self  canvas  color           self canvas   canvas         self id   canvas create oval 10  10  25  25  fill color          self canvas move self id  245  100      def draw self           pass  ball   Ball canvas   quot red quot    tk mainloop    However  when tried the next step in this program  making the ball move by time   the book am reading from  says to do the following  So I changed the draw function to  def draw self       self canvas move self id  0  -1   and add the following code to my program  while 1      ball draw       tk update idletasks       tk update       time sleep 0 01   But I noticed that adding this block of code  made the use of tk mainloop   useless  since everything would show up even without it    At this moment I should mention that my book never talks about tk mainloop    maybe because it uses Python 3  but I learned about it searching the web since my programs didn t work by copying book s code  So I tried doing the following that would not work    while 1      ball draw       tk mainloop       time sleep 0 01   What s going on  What does tk mainloop    What does tk update idletasks   and tk update   do and how that differs from tk mainloop    Should I use the above loop tk mainloop    or both in my programs

User · Answer

I m using an MVC   MVA design pattern  with multiple types of  views    One type is a  GuiView   which is a Tk window   I pass a view reference to my window object which does things like link buttons back to view functions  which the adapter   controller class also calls      In order to do that  the view object constructor needed to be completed prior to creating the window object   After creating and displaying the window  I wanted to do some initial tasks with the view automatically   At first I tried doing them post mainloop    but that didn t work because mainloop   blocked     As such  I created the window object and used tk update   to draw it   Then  I kicked off my initial tasks  and finally started the mainloop   import Tkinter as tk  class Window tk Frame       def   init   self  master None  view None            tk Frame   init    self  master           self view    view                    Setup window linking it to the view         class GuiView  MyViewSuperClass         def open  self            self tkRoot    tk Tk           self window    Window  master None  view self           self window  pack           self refresh           self onOpen           self tkRoot  mainloop                 def onOpen  self                        Do some initial tasks             def refresh  self                    self tkRoot  update

User · Answer

while 1      root update         is  very   roughly similar to   root mainloop     The difference is  mainloop is the correct way to code and the infinite loop is subtly incorrect  I suspect  though  that the vast majority of the time  either will work  It s just that mainloop is a much cleaner solution  After all  calling mainloop is essentially this under the covers    while the window has not been destroyed        wait until the event queue is not empty       event   event queue pop       event handle         which  as you can see  isn t much different than your own while loop  So  why create your own infinite loop when tkinter already has one you can use   Put in the simplest terms possible  always call mainloop as the last logical line of code in your program  That s how Tkinter was designed to be used

User · Answer

tk mainloop   blocks   It means that execution of your Python commands halts there   You can see that by writing  while 1      ball draw       tk mainloop       print  quot hello quot      NEW CODE     time sleep 0 01   You will never see the output from the print statement  Because there is no loop  the ball doesn t move  On the other hand  the methods update idletasks   and update   here  while True      ball draw       tk update idletasks       tk update       do not block  after those methods finish  execution will continue  so the while loop will execute over and over  which makes the ball move  An infinite loop containing the method calls update idletasks   and update   can act as a substitute for calling tk mainloop     Note that the whole while loop can be said to block just like tk mainloop   because nothing after the while loop will execute  However  tk mainloop   is not a substitute for just the lines  tk update idletasks   tk update    Rather  tk mainloop   is a substitute for the whole while loop  while True      tk update idletasks       tk update    Response to comment  Here is what the tcl docs say   Update idletasks This subcommand of update flushes all currently-scheduled idle events from Tcl s event queue  Idle events are used to postpone processing until    there is nothing else to do     with the typical use case for them being Tk s redrawing and geometry recalculations  By postponing these until Tk is idle  expensive redraw operations are not done until everything from a cluster of events  e g   button release  change of current window  etc   are processed at the script level  This makes Tk seem much faster  but if you re in the middle of doing some long running processing  it can also mean that no idle events are processed for a long time  By calling update idletasks  redraws due to internal changes of state are processed immediately   Redraws due to system events  e g   being deiconified by the user  need a full update to be processed   APN As described in Update considered harmful  use of update to handle redraws not handled by update idletasks has many issues  Joe English in a comp lang tcl posting describes an alternative   So update idletasks   causes some subset of events to be processed that update   causes to be processed  From the update docs   update  idletasks  The update command is used to bring the application    up to date    by entering the Tcl event loop repeatedly until all pending events  including idle callbacks  have been processed  If the idletasks keyword is specified as an argument to the command  then no new events or errors are processed  only idle callbacks are invoked  This causes operations that are normally deferred  such as display updates and window layout calculations  to be performed immediately  KBK  12 February 2000  -- My personal opinion is that the  update  command is not one of the best practices  and a programmer is well advised to avoid it  I have seldom if ever seen a use of  update  that could not be more effectively programmed by another means  generally appropriate use of event callbacks  By the way  this caution applies to all the Tcl commands  vwait and tkwait are the other common culprits  that enter the event loop recursively  with the exception of using a single  vwait  at global level to launch the event loop inside a shell that doesn t launch it automatically  The commonest purposes for which I ve seen  update  recommended are   Keeping the GUI alive while some long-running calculation is executing  See Countdown program for an alternative  2  Waiting for a window to be configured before doing things like geometry management on it  The alternative is to bind on events such as  that notify the process of a window s geometry  See Centering a window for an alternative   What s wrong with update  There are several answers  First  it tends to complicate the code of the surrounding GUI  If you work the exercises in the Countdown program  you ll get a feel for how much easier it can be when each event is processed on its own callback  Second  it s a source of insidious bugs  The general problem is that executing  update  has nearly unconstrained side effects  on return from  update   a script can easily discover that the rug has been pulled out from under it  There s further discussion of this phenomenon over at Update considered harmful          Is there any chance I can make my program work without the while loop   Yes  but things get a little tricky   You might think something like the following would work  class Ball      def   init   self  canvas  color           self canvas   canvas         self id   canvas create oval 10  10  25  25  fill color          self canvas move self id  245  100       def draw self           while True             self canvas move self id  0  -1   ball   Ball canvas   quot red quot   ball draw   tk mainloop    The problem is that ball draw   will cause execution to enter an infinite loop in the draw   method  so tk mainloop   will never execute  and your widgets will never display    In gui programming  infinite loops have to be avoided at all costs in order to keep the widgets responsive to user input  e g  mouse clicks  So  the question is  how do you execute something over and over again without actually creating an infinite loop   Tkinter has an answer for that problem  a widget s after   method  from Tkinter import   import random import time  tk   Tk   tk title    quot Game quot  tk resizable 0 0  tk wm attributes  quot -topmost quot   1   canvas   Canvas tk  width 500  height 400  bd 0  highlightthickness 0  canvas pack    class Ball      def   init   self  canvas  color           self canvas   canvas         self id   canvas create oval 10  10  25  25  fill color          self canvas move self id  245  100       def draw self           self canvas move self id  0  -1          self canvas after 1  self draw     time delay  method to execute             ball   Ball canvas   quot red quot   ball draw     Changed per Bryan Oakley s comment tk mainloop    The after   method doesn t block  it actually creates another thread of execution   so execution continues on in your python program after after   is called  which means tk mainloop   executes next  so your widgets get configured and displayed   The after   method also allows your widgets to remain responsive to other user input   Try running the following program  and then click your mouse on different spots on the canvas  from Tkinter import   import random import time  root   Tk   root title    quot Game quot  root resizable 0 0  root wm attributes  quot -topmost quot   1   canvas   Canvas root  width 500  height 400  bd 0  highlightthickness 0  canvas pack    class Ball      def   init   self  canvas  color           self canvas   canvas         self id   canvas create oval 10  10  25  25  fill color          self canvas move self id  245  100           self canvas bind  quot  lt Button-1 gt  quot   self canvas onclick          self text id   self canvas create text 300  200  anchor  se           self canvas itemconfig self text id  text  hello        def canvas onclick self  event           self canvas itemconfig              self text id               text  quot You clicked at          quot  format event x  event y                 def draw self           self canvas move self id  0  -1          self canvas after 50  self draw             ball   Ball canvas   quot red quot   ball draw     Changed per Bryan Oakley s comment  root mainloop

[python] Tkinter understanding mainloop

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