Stacking Circles - Python Pygame

import pygame
import pygame.gfxdraw
from random import uniform as uRnd
from random import randint as rnd
import math, time

FPS = 120
W, H = 1280, 720
HW, HH = int(W / 2), int(H / 2)

WHITE = [255, 255, 255]
BLACK = [  0,   0,   0]

# game time
class gTime:
    def __init__(s, fps): # input = frames per second
        s.fps = fps
        s.tpf = 1 / fps * 1000000000
        s.time = 0
        s.timeStamp = time.time_ns()

    def tick(s):
        while time.time_ns() - s.timeStamp < s.tpf:
            pass
        s.timeStamp = time.time_ns()
        s.time += 1

pygame.init()
DS = pygame.display.set_mode((W, H))
TIME = gTime(FPS)

GRAVITY = 0.1

BARRIER_X1 = HW - 50
BARRIER_X2 = HW + 50

class circles:
    class circle:
        def __init__(s, pos):
            s.x = pos[0]
            s.y = pos[1]
            s.dy = 0
            s.updated = False

    def __init__(s):
        s.circles = []
        s.bx = [BARRIER_X1, BARRIER_X2]

    def add(s):
        s.circles += [s.circle([HW - uRnd(-3, 3), 0])]

    def draw(s):
        for c in s.circles:
            pygame.draw.circle(DS, WHITE, (int(c.x), int(c.y)), 10)

    def update(s):
        for c1 in s.circles:
            x1 = c1.x
            y1 = c1.y

            for c2 in s.circles:
                if c2 == c1: continue

                x2 = c2.x
                y2 = c2.y

                x = x2 - x1
                y = y2 - y1

                d = math.hypot(x, y)

                if d <= 20:
                    c2.x += x / d * 2
                    c2.y += y / d * 2
                    g = c2.dy / d
                    c2.dy -= g

            c1.y += c1.dy
            c1.dy += GRAVITY

            if c1.y >= H - 10:
                c1.y = H - 10
                c1.dy = 0
            if c1.x < s.bx[0] + 10:
                c1.x = s.bx[0] + 10
            if c1.x > s.bx[1] - 10:
                c1.x = s.bx[1] - 10
C = circles()

ADD_CIRCLE_TIME = FPS
timer = ADD_CIRCLE_TIME

while True:
    e = pygame.event.get()
    if pygame.key.get_pressed()[pygame.K_ESCAPE]: break

    DS.fill(BLACK)

    if timer:
        timer -= 1
        if not timer:
            timer = ADD_CIRCLE_TIME
            C.add()

    C.draw()
    C.update()
    if len(C.circles) > 100:
        C.bx = [0, W]
    else:
        pygame.draw.line(DS, WHITE, [C.bx[0], 0], [C.bx[0], H])
        pygame.draw.line(DS, WHITE, [C.bx[1], 0], [C.bx[1], H])

    pygame.display.update()
    TIME.tick()

pygame.quit()