Rain of Bombs (Python recipe) by Stephen Chappell

ActiveState Code (http://code.activestate.com/recipes/502251/)

import random           # FOR RANDOM BEGINNINGS
from Tkinter import *   # ALL VISUAL EQUIPMENT

WIDTH = 500             # OF SCREEN IN PIXELS
HEIGHT = 500            # OF SCREEN IN PIXELS
BALLS = 20              # IN SIMULATION
WALL = 50               # FROM SIDE IN PIXELS
WALL_FORCE = 500        # ACCELERATION PER MOVE
SPEED_LIMIT = 5000      # FOR ball VELOCITY
BALL_RADIUS = 15        # FOR ballS IN PIXELS
OFFSET_START = 100      # FROM WALL IN PIXELS
FRAMES_PER_SEC = 40     # SCREEN UPDATE RATE
COLORS = 'red', 'orange', 'yellow', \
         'green', 'blue', 'purple'

################################################################################

def main():
    # Start the program.
    initialise()
    mainloop()

def initialise():
    # Setup simulation variables.
    global active
    active = False
    build_balls()
    build_graph()

def build_graph():
    # Build GUI environment.
    global graph, left, top
    root = Tk()
    root.resizable(False, False)
    root.title('Rain of Bombs')
    left = (root.winfo_screenwidth() - WIDTH) / 2
    top = (root.winfo_screenheight() - HEIGHT) / 2
    root.geometry('%dx%d+%d+%d' % (WIDTH, HEIGHT, left, top))
    root.bind_all('<Escape>', lambda event: event.widget.quit())
    root.bind('<Configure>', window_move)
    graph = Canvas(root, width=WIDTH, height=HEIGHT, background='white')
    graph.after(1000 / FRAMES_PER_SEC, update)
    graph.after(1000, activate)
    graph.bind('<1>', change)
    graph.pack()

def change(event):
    try:
        ball = balls[int(graph.gettags(graph.find_withtag(CURRENT))[0])]
        ball.color = (ball.color + 1) % len(COLORS)
    except:
        pass

def activate():
    # Active window_move event.
    global active
    active = True

def window_move(event):
    # Respond to movements.
    global left, top
    if active:
        diff = TwoD(left - event.x, top - event.y)
        for ball in balls:
            if HEIGHT - WALL - 2 < ball.position.y and top > event.y:
                ball.velocity.y -= (1000 * (top - event.y))
            ball.position += diff
        left, top = event.x, event.y
    
def update():
    # Main simulation loop.
    graph.after(1000 / FRAMES_PER_SEC, update)
    draw()
    move()

def draw():
    graph.delete(ALL)
    # Draw sides.
    graph.create_rectangle((0, 0, WALL - BALL_RADIUS, HEIGHT), fill='light green')
    graph.create_rectangle((WIDTH - WALL + BALL_RADIUS, 0, WIDTH, HEIGHT), fill='light green')
    # Draw floor.
    y = HEIGHT - WALL + BALL_RADIUS + 2
    graph.create_line((WALL - BALL_RADIUS, y, WIDTH - WALL + BALL_RADIUS, y), fill='blue', width=3)
    # Draw all balls.
    for n, ball in enumerate(balls):
        x1 = ball.position.x - BALL_RADIUS
        y1 = ball.position.y - BALL_RADIUS
        x2 = ball.position.x + BALL_RADIUS
        y2 = ball.position.y + BALL_RADIUS
        graph.create_oval((x1, y1, x2, y2), fill=COLORS[ball.color], tag=n)
        graph.create_text(ball.position.x, ball.position.y, text=str(n+1), tag=n)
    graph.update()

def move():
    # Move all balls.
    for force in simulate_wall, simulate_gravity, simulate_friction:
        for ball in balls:
            force(ball)
    for ball in balls:
        ball.update_velocity(balls)
    for ball in balls:
        ball.move()

def simulate_wall(ball):
    # Create viewing boundaries.
    if ball.position.x < WALL:
        ball.velocity.x += WALL_FORCE
    elif ball.position.x > WIDTH - WALL:
        ball.velocity.x -= WALL_FORCE
    
    if ball.position.y >= HEIGHT - WALL:
        ball.velocity.y *= -1
        ball.position.y = HEIGHT - WALL

def simulate_gravity(ball):
    # Create a pull.
    ball.velocity.y += 50

def simulate_friction(ball):
    # Slow velocity down.
    ball.velocity *= .9

def limit_speed(ball):
    # Limit ball speed.
    if ball.velocity.mag() > SPEED_LIMIT:
        ball.velocity /= ball.velocity.mag() / SPEED_LIMIT

def build_balls():
    # Create balls variable.
    global balls
    balls = tuple(Ball(WIDTH, HEIGHT, OFFSET_START, FRAMES_PER_SEC) for ball in xrange(BALLS))

################################################################################

# TWO DIMENTIONAL VECTOR CLASS

class TwoD:

    def __init__(self, x, y):
        self.x = float(x)
        self.y = float(y)

    def __repr__(self):
        return 'TwoD(%s, %s)' % (self.x, self.y)

    def __add__(self, other):
        return TwoD(self.x + other.x, self.y + other.y)

    def __sub__(self, other):
        return TwoD(self.x - other.x, self.y - other.y)

    def __mul__(self, other):
        return TwoD(self.x * other, self.y * other)

    def __div__(self, other):
        return TwoD(self.x / other if other else self.x, self.y / other if other else self.y)

    def __iadd__(self, other):
        self.x += other.x
        self.y += other.y
        return self

    def __isub__(self, other):
        self.x -= other.x
        self.y -= other.y
        return self

    def __imul__(self, other):
        self.x *= other
        self.y *= other
        return self

    def __idiv__(self, other):
        self.x /= other
        self.y /= other
        return self

    def mag(self):
        return ((self.x ** 2) + (self.y ** 2)) ** 0.5

################################################################################

# BALL IMPLEMENTATION CLASS

class Ball:

    def __init__(self, width, height, offset, move_divider):
        self.velocity = TwoD(0, 0)
        self.position = TwoD(*(-offset if random.randint(0, 1) else width + offset, random.randint(1, height)))
        self.move_divider = move_divider * 5
        self.color = 0

    def update_velocity(self, balls):
        vector = TwoD(0, 0)
        for ball in balls:
            if ball is not self:
                if (self.position - ball.position).mag() < (BALL_RADIUS * 2 + 2):
                    vector -= (ball.position - self.position)
        self.__temp = vector * self.velocity.mag() / vector.mag() * 10

    def move(self):
        self.velocity += self.__temp
        limit_speed(self)
        self.position += self.velocity / self.move_divider

################################################################################

# Execute the simulation.
if __name__ == '__main__':
    main()