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)