python game: lives_left drops too fast and game is over unexpectedly - pygame

Question

Introduction: Different letters fall down from the top of the screen after a time interval and the letter will vanish while you strike the corresponding key on the keyboard. The x position of each letter is random. Game is over when more than 10 letters have reached the bottom.
Progress:I have implemented some functions such as generating random falling letters and eliminating letters by striking the corresponding keys.
Problem: Game should be over when more than 10 letters had reached the bottom. However, after running the codes, game is immediately over while the first falling letter has reached the bottom. I suppose there's something wrong with 'check_letter_bottom()' method in game_functions.py since it is observed that lives_left drops quickly from 10 to 0. What corrections should I make?
Here are my codes:
alphabet_zoo.py
import pygame
import time
from pygame.locals import *
from settings import Settings
import game_functions as gf
from game_stats import GameStats
def run_game():
pygame.init()
az_settings =Settings()
screen = pygame.display.set_mode((0,0), RESIZABLE)
pygame.display.set_caption("Alphabet Zoo")
stats = GameStats(az_settings)
letters = pygame.sprite.Group()
start = time.time()
sleepTime = 3
while True:
now = time.time()
gf.check_events(letters)
if stats.game_active:
print(stats.lives_left)
letters.update()
gf.update_screen(az_settings, stats, screen, letters)
if now - start >sleepTime:
gf.letter_generator(az_settings ,screen, letters)
start = now
run_game()
settings.py
class Settings():
def __init__(self):
self.bg_color = (0, 0, 0)
self.letter_speed_factor = 10
self.lives_limit = 10
game_functions.py
import sys
import pygame
from letter import Letter
def letter_generator(az_settings, screen, letters):
new_letter = Letter(az_settings, screen)
letters.add(new_letter)
def check_events(letters):
for event in pygame.event.get():
if event.type == pygame.QUIT:
sys.exit()
elif event.type == pygame.KEYDOWN:
for ltr in letters:
if ltr.ascii == event.key:
letters.remove(ltr)
def letter_fallen(stats):
if stats.lives_left > 0:
stats.lives_left -= 1
else:
stats.game_active = False
def check_letter_bottom(screen, letters, stats):
screen_rect = screen.get_rect()
for ltr in letters.sprites():
if ltr.rect.bottom >= screen_rect.bottom: # there might be some problems
letter_fallen(stats)
def update_screen(az_settings, stats, screen, letters):
screen.fill(az_settings.bg_color)
check_letter_bottom(screen, letters, stats)
letters.draw(screen)
letters.update()
pygame.display.flip()
letter.py
import pygame
import random
from pygame.sprite import Sprite
class Letter(Sprite):
def __init__(self, az_settings, screen):
super().__init__()
self.screen = screen
self.az_settings = az_settings
a = random.randint(97, 123)
c = chr(a)
name = c.upper() + '.png'
b = 'images/' + name
print(b)
self.image = pygame.image.load(b)
self.ascii = a
self.rect = self.image.get_rect()
self.screen_rect = screen.get_rect()
self.rect.centerx = random.randint(0, self.screen_rect.right)
self.rect.top = self.screen_rect.top
self.center = float(self.rect.centerx)
def update(self):
if self.rect.bottom < self.screen_rect.bottom:
self.rect.centery += self.az_settings.letter_speed_factor
game_stats.py
class GameStats():
def __init__(self, az_settings):
self.az_settings = az_settings
self.reset_stats()
self.game_active = True
def reset_stats(self):
self.lives_left = self.az_settings.lives_limit

Answer 1

If you want to display the letters after scoring them, you could remove them from the letters group and add them to a different group.
So in your run_game() function, add scored_letters = pygame.sprite.Group().
Then in check_letter_bottom(…)
def check_letter_bottom(screen, letters, scored_letters, stats):
screen_rect = screen.get_rect()
for ltr in letters.sprites():
if ltr.rect.bottom >= screen_rect.bottom:
letter_fallen(stats)
letters.remove(ltr)
scored_letters.add(ltr)
Then include scored_letters.draw(screen) in update_screen(…). Probably before letters.draw() so falling letters are on top.

Answer 2

I have found it is actually simple to solve my problem by adding ltr.rect.bottom = screen_rect.bottom before letter_fallen(stats) and making a little change of the if statement(> instead of >=). It's like this:
def check_letter_bottom(screen, letters, stats):
screen_rect = screen.get_rect()
for ltr in letters.sprites():
if ltr.rect.bottom > screen_rect.bottom:
ltr.rect.bottom = screen_rect.bottom
letter_fallen(stats)