[Python] Raindrop is not going down but repeating itself [duplicate]

I am doing exercise from a python book with Pygame, and I need to make the raindrop with x-coordinate and y-coordinate 0 fall down from top to bottom. But it keeps repeating itself with the y-coordinate changing, just like the picture shows. I have no idea how to deal with it since there is no error.

Version: Pygame 2.6.0 (SDL 2.28.4, Python 3.11.9)

raindrop.py

import sys

import pygame
from pygame.sprite import Group

from rain import Rain

def rain_drop():
# Initialize a pygame screen
pygame.init()
screen = pygame.display.set_mode((1000, 600))
pygame.display.set_caption('Rain Drop')

# Create a raindrop
rain = Rain(screen)

# Main loop
while True:
# Watch for keyboard and mouse event
for event in pygame.event.get():
if event.type == pygame.QUIT:
sys.exit()
if event.type == pygame.KEYDOWN:
if event.key == pygame.K_q:
sys.exit()

# Redraw the screen
rain.blitme()
rain.update()
pygame.display.flip()

rain_drop()


rain.py

import pygame
from pygame.sprite import Sprite

class Rain(Sprite):
def __init__(self, screen):
super().__init__()
self.screen = screen
self.image = pygame.image.load('images/raindrop.webp')

self.rect = self.image.get_rect()
self.screen_rect = self.screen.get_rect()

self.rect.x = 0
self.rect.y = 0

self.x = float(self.rect.x)
self.y = float(self.rect.y)

def blitme(self):
self.screen.blit(self.image, self.rect)

def update(self):
self.y += 100
self.rect.y = self.y

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