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import pygame
import sys

pygame.init()

WIDTH = 800
HEIGHT = 600
player_color = (255, 0, 0)
player_position = [400, 300]
player_size = 50

screen = pygame.display.set_mode((WIDTH, HEIGHT))

game_over = False

while not game_over:

    for event in pygame.event.get():
        # очень удобная штука, чтоб посмотреть координаты мыщи
        # print(event)
        if event.type == pygame.QUIT:
            sys.exit()

    if event.type == pygame.KEYDOWN:

        x = player_position[0]
        y = player_position[1]

        if event.key == pygame.K_LEFT:
            x -= 5
        elif event.key == pygame.K_RIGHT:
            x += 5

        player_position = [x, y]
        screen.fill((0, 0, 0,))

    pygame.draw.rect(screen, player_color, (player_position[0], player_position[1], player_size, player_size))
    pygame.display.update()