Python Pygame - Basic Jump

import pygame

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


SIZE = 600
HALF = 300

pygame.init()
DS = pygame.display.set_mode((SIZE, SIZE))


blockSize =  30 # size of the block
blockX = int(HALF - blockSize / 2)
blockY = SIZE - blockSize # starting position of the block
jumpCounter = 0 # keep track of block's jump height
velocityY = 0 # The velocity of the block -1 (jump up), 0 (still), 1 (falling)

while True:
    e = pygame.event.get()

    k = pygame.key.get_pressed()
    if k[pygame.K_ESCAPE]: break # press escape to exit. Window controls don't work

    DS.fill(BLACK)
    pygame.draw.rect(DS, WHITE, (blockX, blockY, blockSize, blockSize))
    pygame.display.update()
    pygame.time.Clock().tick(60) # 60 FPS

    # block can only jump if the space bar is pressed and the jumpCounter is zero and the velocity is zero
    if k[pygame.K_SPACE] and not jumpCounter and not velocityY:
        jumpCounter = 100 # the height block will jump
        velocityY = -1 # jump up

    blockY += velocityY # add the velocity to the block's y position
    if jumpCounter: # if jumpCounter is greater than 0
        jumpCounter -= 1 # decrement the counter by 1
        if not jumpCounter and velocityY == -1: # if the jump counter is zero and the block is jumping up
            velocityY = 1 # change the velocity of the block so that it's falling
            jumpCounter = 100 # change the value of the jump counter back to 100 so the block falls the same distance as it did jumping up
        elif not jumpCounter and velocityY == 1: # if the jump counter is zero and the block is falling
            velocityY = 0 # set the velocity of the block to zero so that it's motionless. The block should now be able to jump again!