Python Pygame - Glowing Particles

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

# timer: returns number of frames since last call to timer.update()
class timer:
    def __init__(s, seconds):
        s.ts = time.time()
        s.seconds = seconds

    def update(s):
        now = time.time()
        duration = now - s.ts
        frames = int(duration / s.seconds)
        if frames >= 1:
            s.ts = now
        return frames

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

        # particle velocity
        s.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(5, 15) # 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, WHITE, 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 -= 0.05

    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, (20, 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)

# 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

# timer is set to update every 0.01 seconds
# so every 0.01 we'll add a new particle
addParticleTimer = timer(0.01)

# particle container
particles = []

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

    # check if timer wants to update...
    if addParticleTimer.update():
        # ...yes? add particle at the mouse pointer location
        particles += [particle(pygame.mouse.get_pos())]

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

    # 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