title: pygame sprites move in constant line
body: <p>I'm trying (badly) to write a very succinct program in pygame like Pong, but I seem to be stuck trying to move my paddles vertically. Moreover, while they do move... they only move in constant line.</p>
  
  <p><a href="https://i.stack.imgur.com/vApnU.jpg" rel="nofollow noreferrer">Example Image</a></p>
  
  <pre><code>#python code
  import pygame, sys
  
  #Screen Width/Height
  screen = pygame.display.set_mode((800,600))
  
  #Background Image
  background_image = pygame.image.load("tennis.jpg").convert()
  screen.blit(background_image, [0,0])
  
  #Frame Rate
  clock = pygame.time.Clock()
  
  #Paddles
  paddle_player1 = pygame.Rect(5,50,5,60)
  paddle_player2 = pygame.Rect(790,50,5,60)
  
  #Process Player Input
  while True:
      for event in pygame.event.get():
          if event.type == pygame.QUIT:
              pygame.quit()
              sys.exit()
  
         player1_up = pygame.key.get_pressed()[pygame.K_w] 
         player1_down = pygame.key.get_pressed()[pygame.K_s]
  
         player2_up = pygame.key.get_pressed()[pygame.K_UP]
         player2_down = pygame.key.get_pressed()[pygame.K_DOWN]
  
  #Updating Game State Logic
          if player1_up:
              paddle_player1.y += -50
          if player1_down:
              paddle_player1.y += 50
          if player2_up:
              paddle_player2.y += -50
          if player2_down:
              paddle_player2.y += 50
  
          if paddle_player1.y &lt; 0:
              paddle_player1.y = 0
          if paddle_player2.y &lt; 0:
              paddle_player2.y = 0
  
      if paddle_player1.y &gt; screen.get_height() - paddle_player1.height:
          paddle_player1.y = screen.get_height() - paddle_player1.height
      if paddle_player2.y &gt; screen.get_height() - paddle_player2.height:
          paddle_player2.y = screen.get_height() - paddle_player2.height
  
  #Rendering
      pygame.draw.rect(screen,(0,0,0), paddle_player1)
      pygame.draw.rect(screen,(0,0,0), paddle_player2)
  
  
     clock.tick(50)
     pygame.display.flip()
  </code></pre>