pygame报错：'teacher_chen' object has no attribute 'rect'

三个文件：
blank.py:

import sys
import pygame
from setting import settings
import heroes
def run_game():
    pygame.init()
    setting=settings()
    screen=pygame.display.set_mode((setting.width,setting.high))
    chenping=heroes.teacher_chen(screen)
    while True:
        for event in pygame.event.get():
            if event.type==pygame.QUIT:
                sys.exit()
        screen.fill(setting.color)
        chenping.blit()
        pygame.display.flip()
run_game()

heroes.py:

import pygame

class teacher_chen():
    def __init__(self,screen):
        self.screen=screen
        self.image=pygame.image.load('photos/chenping.bmp')
        self.screen_rect=screen.get_rect()
        self.rect.centerx=self.screen_rect.centerx  #在这里报错：'teacher_chen' object has no attribute 'rect'
        self.rect.bottom=self.screen_rect.bottom
    def blit(self):
        self.screen.blit(self.image,self.rect)

完整报错信息：
'teacher_chen' object has no attribute 'rect'
File "C:\Users\70707\Documents\python\game_alien\heroes.py", line 8, in init
self.rect.centerx=self.screen_rect.centerx
File "C:\Users\70707\Documents\python\game_alien\blank.py", line 23, in run_game
chenping=heroes.teacher_chen(screen)
File "C:\Users\70707\Documents\python\game_alien\blank.py", line 31, in
run_game()

self.rect 没有rect呀，怎么会有.centerx