Highly simplified version of an FPS game using Python with Pygame library

import pygame
from pygame.locals import *

# Initialize Pygame
pygame.init()

# Game Constants
SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
FPS = 60

# Colors
WHITE = (255, 255, 255)
BLACK = (0, 0, 0)
RED = (255, 0, 0)

# Set up display
screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))
pygame.display.set_caption('Simple FPS Game')

clock = pygame.time.Clock()

# Player properties
player_x = SCREEN_WIDTH // 2
player_y = SCREEN_HEIGHT // 2
player_speed = 5
player_width = 20
player_height = 20

# Game loop
running = True
while running:
    screen.fill(BLACK)

    for event in pygame.event.get():
        if event.type == QUIT:
            running = False

    keys = pygame.key.get_pressed()
    if keys[K_LEFT]:
        player_x -= player_speed
    if keys[K_RIGHT]:
        player_x += player_speed
    if keys[K_UP]:
        player_y -= player_speed
    if keys[K_DOWN]:
        player_y += player_speed

    pygame.draw.rect(screen, RED, (player_x, player_y, player_width, player_height))

    pygame.display.flip()
    clock.tick(FPS)

pygame.quit()