pygame只显示黑屏？

Question

I'm new to pygame and Python in general. I looked up other threads that asked similar questions but it seems that they reached different solutions. Please, someone tell me what is wrong with my code.

import pygame
import sys

pygame.init()

size = (width, height) = (720, 720)

background_color = (0, 255, 0)

screen = pygame.display.set_mode(size)

ball = pygame.image.load("img/pokeball.jpg")
speed = [2, 3]

ballrect = ball.get_rect()

ballrect.move_ip(0, 0)

while True:

    for event in pygame.event.get():
        if event.type == pygame.QUIT: sys.exit()

ballrect = ballrect.move(speed)

if ballrect.left < 0 or ballrect.right > width:
    speed[0] = -speed[0]

if ballrect.top < 0 or ballrect.bottom > height:
    speed[1] = -speed[1]

screen.fill(background_color)
screen.blit(ball, ballrect)
pygame.display.flip()

Answer 1

is working with correct indentation ( 4 spaces indentation in any loop ( while ,...) or if statement) :

import pygame
import sys

pygame.init()

size = (width, height) = (720, 720)

background_color = (0, 255, 0)

screen = pygame.display.set_mode(size)

ball = pygame.image.load("/absolute_path/pokeball.jpg")
speed = [2, 3]

ballrect = ball.get_rect()

ballrect.move_ip(0, 0)

while True:

    for event in pygame.event.get():

        if event.type == pygame.QUIT: sys.exit()

        ballrect = ballrect.move(speed)

        if ballrect.left < 0 or ballrect.right > width:
            speed[0] = -speed[0]

        if ballrect.top < 0 or ballrect.bottom > height:
            speed[1] = -speed[1]

    screen.fill(background_color)
    screen.blit(ball, ballrect)
    pygame.display.flip()

i also used an absolute path for the ball .jpg