Python Pygame Bouncing Balls with Particles

import pygame
from pygame import Rect
from pygame import Vector2
from random import randint, uniform

# particle
class particle:
    def __init__(s, xy, vel): # xy = tuple(x, y)
        # position of the particle
        s.xy = Vector2(xy)

        # particle velocity
        s.vel = vel #Vector2(uniform(-1, 1), uniform(-4.5, -5)) # x vel is betweem -1 & 1 and y vel is between -4.5 and -5

        # particle radius
        s.radius = randint(4, 8) # random integer between 5 & 15

    # draw particle and update position/size
    def update(s):
        global PDS, WHITE # PDS = primary display surface

        # draw particle
        pygame.draw.circle(PDS, (128, 128, 128), s.xy, s.radius)

        # draw glow
        s.glow()

        # add velocity to particle
        s.xy += s.vel

        # add gravity to velocity. negative velocity will slowly become positive
        s.vel.y += GRAVITY

        # decrease radius to give a fizzling out effect
        s.radius -= 0.1

    def glow(s):
        global PDS
        # get rectangular size of the particle. radius * 2
        r = Vector2(s.radius) * 2 # r = (radius * 2, radius * 2)

        # create a surface twice the size of the particle
        gs = pygame.Surface(r * 2)

        # draw a circle with a radius twice the size of the particle in the center of the new surface
        pygame.draw.circle(gs, (60, 20, 60), r, r.x)

        # blit the new larger particle on the primary display with the center of the smaller particle
        # use BLEND_RGB_ADD to increase luminosity if particles overlap
        PDS.blit(gs, s.xy - r, special_flags=pygame.BLEND_RGB_ADD)

class ball:
    def __init__(s, xy, v):
        s.xy = xy
        s.v = v

    def update(s):
        global OBSTICLES, BALL_RADIUS, BALL_RADIUS_V, BALL_RADIUS2_V

        r = Rect(s.xy - BALL_RADIUS_V, BALL_RADIUS2_V)
        nr = Rect(s.xy + s.v - (10, 10), (20, 20))

        for o in OBSTICLES:
            if not o.colliderect(nr): continue

            if nr.right >= o.left and r.right <= o.left:
                r.right = o.left
                s.v.x = -s.v.x
                burst(r.midright, 1, 0)
            if nr.left <= o.right and r.left >= o.right:
                r.left = o.right
                s.v.x = -s.v.x
                burst(r.midleft, 1, 1)
            if nr.bottom >= o.top and r.bottom <= o.top:
                r.bottom = o.top
                s.v.y = -s.v.y
                burst(r.midbottom, 0, 0)
            if nr.top <= o.bottom and r.top >= o.bottom:
                r.top = o.bottom
                s.v.y = -s.v.y
                burst(r.midbottom, 0, 1)
            s.xy = Vector2(r.center)
        pygame.draw.circle(PDS, WHITE, s.xy, BALL_RADIUS)
        s.xy += s.v

def burst(xy, verticies, ricochet_direction):
    global PARTICLES

    for i in range(20):
        if verticies == 0: # horizontal particle burst
            v = Vector2(uniform(-4, -2) if i // 10 else uniform(2, 4), uniform(1, 2) if ricochet_direction else uniform(-2, -1))
        else:
            v = Vector2(uniform(1, 2) if ricochet_direction else uniform(-2, -1), uniform(-4, -2) if i // 10 else uniform(2, 4))
        PARTICLES += [particle(xy, v)]


# initialise pygame and create primary display surface PDS
pygame.init()
PDR = Rect(0, 0, 1280, 720)
PDS = pygame.display.set_mode(PDR.size)

# define some colors
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)

# strength of gravity
GRAVITY = 0.1

# particle container
PARTICLES = []

# obsticles
OBSTICLES = [
    Rect(0, 0, PDR.w, 20),
    Rect(0, 20, 20, PDR.h),
    Rect(20, PDR.h - 20, PDR.w - 20, 20),
    Rect(PDR.w - 20, 20, 20, PDR.h - 40),
    Rect(PDR.w / 3 - 10, 140, 20, PDR.h - 280),
    Rect(-10 + PDR.w - PDR.w / 3, 140, 20, PDR.h - 280),
    Rect(140, PDR.centery - 10, PDR.w / 3 - 270, 20),
    Rect(PDR.w - 300, PDR.centery - 10, PDR.w / 3 - 270, 20)
]

BALL_RADIUS = 10
BALL_RADIUS_V = Vector2(BALL_RADIUS)
BALL_RADIUS2_V = BALL_RADIUS_V * 2
BALLS = [ball(Vector2(PDR.center), Vector2(5).rotate(d)) for d in range(0, 360, 20)]

exitDemo = False
pause = False # press p to pause
while not exitDemo:

    # handle the events
    events = pygame.event.get()
    for event in events:
        if event.type == pygame.QUIT or (event.type == pygame.KEYDOWN and event.key == pygame.K_ESCAPE):
            exitDemo = True
        if event.type == pygame.KEYDOWN and event.key == pygame.K_p:
            pause = 1 - pause

    if pause: continue

    # clear the PDS to black
    PDS.fill(BLACK)

    for o in OBSTICLES:
        pygame.draw.rect(PDS, (60, 20, 60), o, 2)

    for b in BALLS:
        b.update()

    # create a container to hold particles that need deleting
    killParticles = []
    for p in PARTICLES:
        p.update() # draw and update postion/size of the particle

        # if the particle is outside of the PDS or the radius is less than zero ie no longer visible
        # then add it the killparticles pile
        if p.xy.y >= PDR.bottom or p.radius <= 0:
            killParticles += [p]

    # delete particles from particle container
    for p in killParticles:
        PARTICLES.remove(p)

    # update the PDS
    pygame.display.update()
    pygame.time.Clock().tick(120) # limit FPS to 120