space-invaders game on python

Question

so I was making this space invader game on pygame and now I wanted to add levels to the game so I made a def and put the entire code inside it, but now when I run it works but it does not shoot the bullet. Ive looked around the code again and again but I'm not able to find what/where the mistake is.

#here is the code

 import pygame
    import random
    import math
    from pygame import mixer
    import time
    
    
    
    pygame.init()
    
    screen = pygame.display.set_mode((800, 600))
    
    pygame.display.set_caption("space invaders")
    icon = pygame.image.load("rocket.png")
    pygame.display.set_icon(icon)
    
    def lvl2():
    
        bg = pygame.image.load("background.png")
    
        mixer.music.load("background.wav")
        mixer.music.play(-1)
    
        #player
        pimg = pygame.image.load("space-invaders.png")
        Px = 370
        Py = 480
        Px_change = 0
    
        #enemy
        eimg = []
        ex = []
        ey = []
        ex_change = []
        ey_change = []
        num_of_enemies = 8
    
        for i in range(num_of_enemies):
            eimg.append(pygame.image.load("alien (1).png"))
            ex.append(random.randint(0, 736))
            ey.append(random.randint(50, 150))
            ex_change.append(2)
            ey_change.append(50)
    
        #bullet
        bimg = pygame.image.load("bullet.png")
        bx = 0
        by = 480
        bx_change = 0
        by_change = 10
        b_state = "ready"
    
        #score
        score_value = 0
        font = pygame.font.Font('freesansbold.ttf', 32)
    
        over_font = pygame.font.Font('freesansbold.ttf', 64)
    
        textx = 10
        texty = 10
    
    
        def show_score(x, y):
            score = font.render("score : "+ str(score_value), True, (0,225,0))
            screen.blit(score, (x, y))
    
        def game_over_text():
            over_text = over_font.render("GAMEOVER", True, (0,225,0))
            screen.blit(over_text, (200, 250))
    
        def player(x,y):
            screen.blit(pimg, (x, y))
    
        def enemy(x,y,i):   
            screen.blit(eimg[i], (x, y))
    
        def fire(x, y):
            global b_state
            b_state = "fire"
            screen.blit(bimg, (x+16, y+10))
    
        def collision(ex, ey, bx, by):
            distance = math.sqrt((math.pow(ex-bx,2)) + (math.pow(ey-by,2)))
            if distance <= 27:
                return True
            else:
                return False
    
        running = True
        while running:
            screen.fill((0, 0, 0))
    
            screen.blit(bg,(0,0))
    
            for event in pygame.event.get():
                if event.type == pygame.QUIT: 
                    running = False
    
                if event.type == pygame.KEYDOWN:
                    if event.key == pygame.K_a or event.key == pygame.K_LEFT:
                        Px_change = -4
    
                    if event.key == pygame.K_d or event.key == pygame.K_RIGHT:
                        Px_change = +4
    
                    if event.key == pygame.K_SPACE:
                        if b_state is "ready":
                            b_sound = mixer.Sound("laser.wav")
                            b_sound.play()
                            bx = Px
                            fire(bx, by)
    
                if event.type == pygame.KEYUP:
                    if event.key == pygame.K_a or event.key == pygame.K_d or event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:
                        Px_change = 0
    
    
            Px += Px_change
    
            if Px <= 0:
                Px = 0
    
            if Px >= 735:
                Px = 735
    
            #game over
    
    
            for i in range(num_of_enemies):
            
                if ey[i] > 440:
                    for j in range(num_of_enemies):
                        ey[j] = 2000
                    game_over_text()
                    #pygame.QUIT
                    break
    
                ex[i] += ex_change[i]
    
                if ex[i] <= 0:
                    ex_change[i] = 2
                    ey[i] += ey_change[i]
    
                elif ex[i] >= 736:
                    ex_change[i] = -2
                    ey[i] += ey_change[i]
    
                #colllison
                coll = collision(ex[i],ey[i],bx,by)
                if coll:
                    by = 480
                    b_state = "ready"
                    score_value += 1
                    kill_sound = mixer.Sound("explosion.wav")
                    kill_sound.play()
                    ex[i] = random.randint(0, 736)
                    ey[i]= random.randint(50, 150)
    
                enemy(ex[i], ey[i], i)
    
            
            if by <= 0:
                by = 480
                b_state = "ready"
    
            if b_state is "fire":
                fire(bx, by)
                by -= by_change
    
            player(Px, Py)
            show_score(textx, texty)
    
            pygame.display.update()
    
    lvl2()

if you know how to solve this problem please do help me out

thankyou