PyGame - Very Basic Jumper

#!/usr/bin/env python
#created by Kris Occhipinti (AKA Metalx1000)
#http://filmsbykris.com
#Oct. 16th 2010
#GPLv3  http://www.gnu.org/licenses/gpl-3.0.html

import pygame, sys
from pygame.locals import *

pygame.init()

screen = pygame.display.set_mode((600,400))

image = pygame.image.load("tux.png")
tux = pygame.transform.scale(image, (64,64))

clock = pygame.time.Clock()

gravity = 10
x = -100
y = 300
yr = 0
yl = 0
speed = 4
force = 0
jmp = 0

def jump():
    global x,y,force
    if force > 0:
       x -= force
       force -= 1

def move_right():
    global y,speed
    y += speed

def move_left():
    global y,speed
    y -= speed

while 1:
    clock.tick(60)
    x+=gravity
    screen.fill((0,0,0))
    screen.blit(tux,(y,x))
    pygame.display.update()
    if x > 400-64:
       x = 400-64

    for event in pygame.event.get():
       if event.type == pygame.QUIT:
           sys.exit()
       elif event.type == KEYDOWN and event.key == K_ESCAPE:
           sys.exit()
       elif event.type == KEYDOWN and event.key == K_SPACE:
           force = 25
       elif event.type == KEYDOWN and event.key == K_RIGHT:
           yr = 1
       elif event.type == KEYUP and event.key == K_RIGHT:
           yr = 0
       elif event.type == KEYDOWN and event.key == K_LEFT:
           yl = 1
       elif event.type == KEYUP and event.key == K_LEFT:
           yl = 0

    jump()
    if yr == 1:
       move_right()
    if yl == 1:
       move_left()