Elements in Pygame game are flickering

Question

In my newly created, unfinished snake game, all the objects that are displayed on the screen are flickering. I saw some similar questions, but the solution always seemed to be that someone called the pygame.display.flip() function twice, but I don't think I have done the same thing because I only use the function once per method. I follow this tutorial on youtube and I have the exact same code (I think) but his game isn't flickering https://www.youtube.com/watch?v=8dfePlONtls&t=3940s .This is my Code btw:

import pygame
from pygame.locals import *
import time
import random


SIZE = 40


class Apple:
    def __init__(self, parent_screen):
        self.image = pygame.image.load("resources/apple.jpg").convert()
        self.parent_screen = parent_screen
        self.x = SIZE*3
        self.y = SIZE*3

    def draw(self):
        self.parent_screen.blit(self.image, (self.x, self.y))
        pygame.display.flip()

    def move(self):
        self.x = random.randint(0, 24)*SIZE
        self.y = random.randint(0, 19)*SIZE


class Snake:
    def __init__(self, parent_screen, length):
        self.length = length
        self.parent_screen = parent_screen
        self.block = pygame.image.load("resources/block.jpg").convert()
        self.x = [SIZE]*length
        self.y = [SIZE]*length
        self.direction = "down"

    def increas_length(self):
        self.length += 1
        self.x.append(-1)
        self.y.append(-1)

    def move_left(self):
        self.direction = "left"

    def move_right(self):
        self.direction = "right"

    def move_up(self):
        self.direction = "up"

    def move_down(self):
        self.direction = "down"

    def draw(self):
        self.parent_screen.fill((110, 110, 5))
        for i in range(self.length):
            self.parent_screen.blit(self.block, (self.x[i], self.y[i]))
        pygame.display.flip()

    def walk(self):

        for i in range(self.length-1, 0, -1):
            self.x[i] = self.x[i - 1]
            self.y[i] = self.y[i - 1]

        if self.direction == "left":
            self.x[0] -= SIZE

        if self.direction == "right":
            self.x[0] += SIZE

        if self.direction == "up":
            self.y[0] -= SIZE

        if self.direction == "down":
            self.y[0] += SIZE

        self.draw()


class Game:
    def __init__(self):
        pygame.init()
        self.surface = pygame.display.set_mode((1000, 800))
        self.surface.fill((110, 110, 5))
        self.snake = Snake(self.surface, 2)
        self.snake.draw()
        self.apple = Apple(self.surface)
        self.apple.draw()

    def is_collision(self, x1, y1, x2, y2):
        if x1 >= x2 and x1 < x2 + SIZE:
            if y1 >= y2 and y1 < y2 + SIZE:
                return True

        return False

    def play(self):
        self.snake.walk()
        self.apple.draw()
        self.display_score()
        pygame.display.flip()
        if self.is_collision(self.snake.x[0], self.snake.y[0], self.apple.x, self.apple.y):
            self.snake.increas_length()
            self.apple.move()

    def display_score(self):
        font = pygame.font.SysFont("arial", 30)
        score = font.render(f"Score: {self.snake.length}", True, (255, 255, 255))
        self.surface.blit(score, (800, 10))

    def run(self):
        running = True

        while running:
            for event in pygame.event.get():
                if event.type == KEYDOWN:
                    if event.key == K_ESCAPE:
                        running = False
                    if event.key == K_UP:
                        self.snake.move_up()

                    if event.key == K_DOWN:
                        self.snake.move_down()

                    if event.key == K_LEFT:
                        self.snake.move_left()

                    if event.key == K_RIGHT:
                        self.snake.move_right()

                elif event.type == QUIT:
                    running = False

            self.play()

            time.sleep(0.2)


if __name__ == "__main__":
    game = Game()
    game.run()

Sorry for my bad English and this bad written question.I still hope you can help me!