In alien invasion game my code for firing an bullet and loading alien doesn't show output after running?

Here's the output of the game.

I am taking reference of the ebook "Python Crash Course", I use Pycharm IDE and have also installed python and pygame in it, the game worked fine till the left right movement of the ship. Also I have another problem about loading the alien image in the game I have saved the image in the PYGAME directory inside images folder as an bitmap image as the pygame accept BMP images easily. After running the program the alien ship doesn't seems to appear in the game. I have also added the code for ship(not alien ship). Please do help me about the firing bullets from the ship. I have used five classes till now like ship.py, bullet.py, alien.py, settings.py, game functions.py

Here is the code for firing bullets class

import pygame as pg
from pygame.sprite import Sprite


class Bullet(Sprite):
    def __init__(self, ai_settings, screen, ship):
        super(Bullet, self).__init__()
        self.screen = screen
        self.rect = pg.Rect(0, 0, ai_settings.bullet_width, ai_settings.bullet_height)
        self.rect.centerx = ship.rect.centerx
        self.rect.top = ship.rect.top
        self.y = float(self.rect.y)
        self.color = ai_settings.bullet_color
        self.speed_factor = ai_settings.bullet_speed_factor

    def update(self):
        self.y -= self.speed_factor
        self.rect.y = self.y

    def draw_bullet(self):
        pg.draw.rect(self.screen, self.color, self.rect)

Here is the code of alien class

import pygame as pg
from pygame.sprite import Sprite


class Alien(Sprite):
    def __init__(self, ai_settings, screen):
        super(Alien, self).__init__()
        self.screen = screen
        self.ai_settings = ai_settings
        self.image = pg.image.load('images/alien.bmp')
        self.rect = self.image.get_rect()
        self.rect.x = self.rect.width
        self.rect.y = self.rect.height
        self.x = float(self.rect.x)
    def blitme(self):
        self.screen.blit(self.image, self.rect)

Here is the ship.py class code.

import pygame


class Ship:
    def __init__(self, ai_settings, screen):
        self.screen = screen
        self.ai_settings = ai_settings
        self.image = pygame.image.load('images/ship.bmp')
        self.rect = self.image.get_rect()
        self.screen_rect = screen.get_rect()

        self.rect.centerx = self.screen_rect.centerx
        self.rect.bottom = self.screen_rect.bottom
        self.center = float(self.rect.centerx)
        self.moving_right = False
        self.moving_left = False

    def update(self):
        if self.moving_right:
            if self.moving_right and self.rect.right < self.screen_rect.right:
                self.center += self.ai_settings.ship_speed_factor
            self.rect.centerx += 1
        if self.moving_left:
            if self.moving_left and self.rect.left > 0:
                self.center -= self.ai_settings.ship_speed_factor
            self.rect.centerx -= 1
        self.rect.centerx = self.center

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

Here is the class implementation in the main program Alien Invasion.py

import pygame as pg
from pygame.sprite import Group

import game_functions as gf
from alien import Alien
from settings import Settings
from ship import Ship


def run_game():
    pg.init()
    ai_settings = Settings()
    screen = pg.display.set_mode((ai_settings.scree_width, ai_settings.screen_height))
    pg.display.set_caption('Alien Invasion')
    ship = Ship(ai_settings, screen)
    bullets = Group()
    alien = Alien(ai_settings, screen)
    while True:
        gf.check_events(ai_settings, screen, ship, bullets)
        ship.update()
        gf.update_bullets(bullets)
        gf.update_screen(ai_settings, screen, ship, alien, bullets)


run_game()