Processing_py - Balls criss-crossing

ball.py

class Ball(object):
    
    def __init__(self,_id, _x, _y, _vx=0, _vy=0):
        self.x, self.y = _x, _y
        self.vx, self.vy = _vx, _vy
        self.id = _id
        #store the starting coords of each ball. For relaunching.
        self.startx, self.starty = _x, _y
        self.active = False
        
    def move(self):
        self.x += self.vx
        self.y += self.vy
        
        #check for out of bounds
        if self.x > width or self.y > height or self.x <0 or self.y < 0:
            self.x=self.startx
            self.y=self.starty
        
    def display(self):
        ellipse(self.x, self.y, 20,20)

balls_crossing.py

w, h = 800, 800

from ball import Ball 

num_set = 8 #half the balls are horizontal, the other half will move vertically
num_balls = num_set * 2
hballs = [Ball(id,0,h/2,5,0) for id in range(num_set)] 
vballs = [Ball(id+num_set, w/2,h, 0,-5) for id in range(num_set)]
balls = [None]*(len(hballs)+len(vballs)) #placeholder
balls[::2] = hballs #odd places
balls[1::2] = vballs #even elements
num_active = 0
launch_gap = num_balls
        
def setup():    
    size(w,h)
    background(127)
    smooth()
    noStroke()
    
def draw():
    global balls, num_active
    background(127)
    
    #when to launch?
    if num_active < num_balls:
        if not frameCount%(launch_gap):
            balls[num_active].active = True #launch one more ball
            num_active+=1
            
    #move each ball and show it on the screen
    for b in balls:
        if b.active:
            b.move()
            b.display()
            
    
    

These files are both Python Processing scripts. Save them to as your sketches and launch Processing to see the balls moving across your screen.
https://py.processing.org/