pygame and sprite blur movement - pygame

Question

I am struggling to move the sprite correctly. Instead of smooth move I can see blur move and I do not know how to solve it.
Is there any chance you can point what I do incorrectly ?
My target with it to drop the pizza so it hits the bottom and bounce back and bounc back if it hits the top and again the bottom -> bounce -> top -> bounce etc. etc.
import pygame
gravity = 0.5
class PizzaSprite:
def __init__(self, image, spritepos):
self.image = image
self.spos = spritepos
(x, y) = spritepos
self.posx = x
self.posy = y
self.xvel = 1
self.yvel = 1
print "x ", x
print "y ", y
def draw(self, target_surface):
target_surface.blit(self.image, self.spos)
def update(self):
self.posy -= self.yvel
self.spos = (self.posx, self.posy)
return
def main():
pygame.init()
screen_width = 800
screen_height = 600
x = screen_width
y = screen_height
screen = pygame.display.set_mode((screen_width, screen_height))
wall_image = pygame.image.load("wall.png")
sky_image = pygame.image.load("sky.png")
pizza_image = pygame.image.load("pizza.png")
screen.blit(wall_image,(0,200))
screen.blit(sky_image,(0,0))
all_sprites = []
pizza1 = PizzaSprite(pizza_image, (x/2, y/2))
all_sprites.append(pizza1)
while True:
ev = pygame.event.poll()
if ev.type == pygame.QUIT:
break
for sprite in all_sprites:
sprite.update()
for sprite in all_sprites:
sprite.draw(screen)
pygame.display.flip()
pygame.quit()
main()

Answer 1

in the beginning of your main game while loop add
white = (255,255,255)
screen.fill(white)
let me give you a small analogy of what is happening now,
you have paper and a lot of pizza stickers with the intent to make a flip book. To make the illusion of movement on each piece of paper you place a sticker a little bit lower. The screen.fill command essentially clears the screen with the rgb color value you give it. When you dont fill the screen essentially what you are doing is trying to make that flipbook on one piece of paper. You just keep placing more and more stickers a little bit lower making a blur when what you want is one on each page.
and place
pygame.init()
screen_width = 800
screen_height = 600
x = screen_width
y = screen_height
screen = pygame.display.set_mode((screen_width, screen_height))
wall_image = pygame.image.load("wall.png")
sky_image = pygame.image.load("sky.png")
all outside of your main game loop assuming you wont be making changes to these variables ever in your program it is tedious and inefficient to redefine screen,,x,y,and your two images over and over again if they dont change.
so to sum it all up:
use the screen.fill(white) command to reset the color of your screen
You only need to import pngs and define variables once if they are never going to change and don't need them in your main loop
hope this helps clear things up.