Python Pygame - Colliding Balls

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

class ball:
    def __init__(s, balls):
        global PDR, BALL_RADIUS

        # find a space for the balls
        found_space = False

        if balls:
            while not found_space:
                v = Vector2(randint(0, PDR.w), randint(0, PDR.h))
                for b in balls:
                    if b.v.distance_to(v) <= BALL_RADIUS * 2:
                        continue
                    found_space = True
                    break
        else:
            v = Vector2(randint(0, PDR.w), randint(0, PDR.h))
        s.v = v
        # direction
        s.d = Vector2(0, 1).rotate(randint(0, 359))

class balls:
    def __init__(s, n):
        s.table = []
        for _ in range(n):
            s.table.append(ball(s.table))

    def update(s):
        global PDS, PDR, WHITE, BALL_RADIUS

        for b1 in s.table:
            # check for collision with sides of PDS
            if b1.v.x - BALL_RADIUS <= 0:
                b1.d.x = -b1.d.x
                b1.v.x = BALL_RADIUS
            if b1.v.x + BALL_RADIUS >= PDR.w:
                b1.d.x = -b1.d.x
                b1.v.x = PDR.w - BALL_RADIUS
            if b1.v.y - BALL_RADIUS <= 0:
                b1.d.y = -b1.d.y
                b1.v.y = BALL_RADIUS
            if b1.v.y + BALL_RADIUS >= PDR.h:
                b1.d.y = -b1.d.y
                b1.v.y = PDR.h - BALL_RADIUS

            # check for collision with other balls
            for b2 in s.table:
                if b1 == b2:
                    continue
                if b1.v.distance_to(b2.v) <= BALL_RADIUS * 2:
                    b1.d = b1.d.reflect(b2.v - b1.v)

            # draw the ball to the PDS
            pygame.draw.circle(PDS, WHITE, b1.v, BALL_RADIUS)
            # apply velocity (_.d) to ball
            b1.v += b1.d

pygame.init()
PDR = Rect(0, 0, 1280, 720)
PDS = pygame.display.set_mode(PDR.size)

BLACK = (0, 0, 0)
WHITE = (255, 255, 255)

BALL_RADIUS = 20

T = balls(10)

quit_demo = False
while not quit_demo:
    for e in pygame.event.get():
        if e.type == pygame.KEYUP:
            if e.key == pygame.K_ESCAPE:
                quit_demo = True

    PDS.fill(BLACK)

    T.update()

    pygame.display.update()
    pygame.time.Clock().tick(120)

pygame.quit()