Display a simulation using Tkinter I wrote a module to simulate physics of 2D elastic balls and the community helped me to improve it on this post.

Now I implemented a GUI using Tkinter to display the simulation in a window. 

I'm a beginner in programming GUI and I don't know if my script can be more efficient and/or simpler.

Indeed, I'm not really satisfied by my display function because it includes definitions of other functions dedicated to the buttons commands.

Moreover, when I push the start button twice, I also need to push the pause button twice to stop the simulation. I don't understand this behaviour !

Import modules

import Tkinter as tk
import solver


You'll find the solver module here. This isn't the object of this post. If you've comments or remarks about it, post them on this post I spoke above.

Surrounding functions

def _create_circle(self, x, y, r, **kwargs):
    """Create a circle

    x the abscissa of centre
    y the ordinate of centre
    r the radius of circle
    **kwargs optional arguments
    return the drawing of a circle
    """
    return self.create_oval(x-r, y-r, x+r, y+r, **kwargs)
tk.Canvas.create_circle = _create_circle

def _coords_circle(self, target, x, y, r, **kwargs):
    """Define a circle

    target the circle object
    x the abscissa of centre
    y the ordinate of centre
    r the radius of circle
    **kwargs optional arguments
    return the circle drawing with updated coordinates
    """
    return self.coords(target, x-r, y-r, x+r, y+r, **kwargs)
tk.Canvas.coords_circle = _coords_circle

def create(balls, canvas):
    """Create a drawing item for each solver.Ball object

    balls the list of solver.Ball objects
    canvas the Tkinter.Canvas oject
    return a dictionary with solver.Ball objects as keys and their circle drawings as items
    """
    return {ball: canvas.create_circle(ball.position[0], ball.position[1], ball.radius, fill="white") for ball in balls}

def update(drawing, canvas, step, size):
    """Update the drawing items for a time step

    drawing the dictionary of drawing items
    canvas the Tkinter.Canvas oject
    step the time step
    size the medium size
    """
    balls = drawing.keys()
    solver.solve_step(balls, step, size)
    for ball in balls:
        canvas.coords_circle(drawing[ball], ball.position[0], ball.position[1], ball.radius)
    canvas.update()


display function

def display(balls, step, size):
    """Display the simulation

    balls the list of solver.Ball objects
    step the time step
    size the medium size
    """
    # Instanciate the window, canvas and circle objects
    window = tk.Tk()
    window.poll = True
    canvas = tk.Canvas(window, width=size, height=size, bg="black")
    canvas.pack()
    canvas.focus_set()
    drawing = create(balls, canvas)
    # Define functions to launch and stop the simulation
    def animate():
        """Animate the drawing items"""
        if window.poll:
            update(drawing, canvas, step, size)
            window.after(0, animate)
        else:
            window.poll = True
    def stop():
        """Stop the animation"""
        window.poll = False
    # Define the buttons used to launch and stop the simulation
    start_button = tk.Button(window, text="Start", command=animate)
    stop_button = tk.Button(window, text="Pause", command=stop)
    start_button.pack()
    stop_button.pack()
    # GUI loop
    window.mainloop()


Unit test of display function

# Test this module
if __name__ == "__main__":
    balls = [solver.Ball(20., 20., [40.,40.], [5.,5.]), solver.Ball(10., 10., [480.,480.], [-15.,-15.]), solver.Ball(15., 15., [30.,470.], [10.,-10.])]
    size = 500.
    step = 0.1
    display(balls, step, size)
As I didn't get any answer, I propose the following improvements.

First I didn't change the functions create_circle and coords_circle.

I found a more satisfying solution for the display function. Indeed, I create a class Display and implement the former display function in its __init__ method.

class Display:
    """Define the window used to display a simulation"""

    def __init__(self, balls, step, size):
        """Initialize and launch the display"""
        self.balls = balls
        self.step = step
        self.size = size

        self.window = tk.Tk()
        self.canvas = tk.Canvas(self.window, width=self.size, height=self.size, bg="black")
        self.canvas.pack()
        self.canvas.focus_set()
        self.drawing = self.create()
        self.started = False

        start_button = tk.Button(self.window, text="Start", command=self.start)
        stop_button = tk.Button(self.window, text="Pause", command=self.stop)
        start_button.pack()
        stop_button.pack()

        self.window.mainloop()


In this way, I can define the animate and stop functions as methods of the class Display rather than inside __init__.

The objects balls, step, size, window, canvas, drawing, start_button and stop_button become attributes of the class Display. So I don't need to put them in arguments of create and update methods.

def create(self):
    """Create a drawing item for each solver.Ball object

    return a dictionary with solver.Ball objects as keys and their circle drawings as items
    """
    return {ball: self.canvas.create_circle(ball.position[0], ball.position[1], ball.radius, fill="white") for ball in self.balls}

def update(self):
    """Update the drawing items for a time step"""
    solver.solve_step(self.balls, self.step, self.size)
    for ball in self.balls:
        self.canvas.coords_circle(self.drawing[ball], ball.position[0], ball.position[1], ball.radius)
    self.canvas.update()


Moreover, the start button calls the start method that calls the animate method only if the value of started is False. Thus, we don't have the bad behaviour raised in the question (need to push the pause button twice after pushing the start button twice).

def start(self):
    """Start the animation"""
    if not self.started:
        self.started = True
        self.animate()

def animate(self):
    """Animate the drawing items"""
    if self.started:
        self.update()
        self.window.after(0, self.animate)

def stop(self):
    """Stop the animation"""
    self.started = False


You'll find the complete code here.