I have the following code

import pygame

def draw_bar(surface, position, size, color_border, color_background, color_health, progress):
        pygame.draw.rect(surface, color_background, (*position, *size))
        pygame.draw.rect(surface, color_border, (*position, *size), 1)
        innerPos  = (position[0]+1, position[1]+1)
        innerSize = (int((size[0]-2) * progress), size[1]-2)
        pygame.draw.rect(surface, color_health, (*innerPos, *innerSize))

class Player:
    def __init__(self, image_name, position, window, max_hp):
        self.image = pygame.image.load(image_name).convert_alpha()
        self.image = pygame.transform.scale(self.image, 
                                            (int(self.image.get_width() * 0.5), 
                                             int(self.image.get_height() * 0.5)))
        self.window = window
        self.position = position
        self.rect = self.image.get_rect(center=self.position) 
        self.max_hp = max_hp
        self.current_hp = self.max_hp

    def draw_health_bar(self, surf):
            health_rect = pygame.Rect(0, 0, self.image.get_width(), 10)
            health_rect.midbottom = self.rect.centerx, self.rect.top
            draw_bar(surf, health_rect.topleft, health_rect.size, 
                    (0, 0, 0), (255, 0, 0), (0, 255, 0), self.current_hp / self.max_hp)
            
    def draw(self):
        self.window.fill((0, 0, 0))
        self.window.blit(self.image, (self.position[0] - self.image.get_width()/2, self.position[1] - self.image.get_height()/2))

window = pygame.display.set_mode((300, 300))
character = Player('player.png', [150, 150], window, 80)
run = True
while run:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
    
    damage = int(input('enter number:'))
    character.current_hp -= damage
    
    character.draw()                        #1
    character.draw_health_bar(window)       #2
    pygame.display.flip()                   #3

When it was executed, I could only see a black window.

If I added the additional lines (#1, #2 and #3) BEFORE the While loop, the drawn images were visible. However, when I entered 10 in the input prompt (thus, current_hp was reduced to 70), the green bar was not updated (to be narrower to reveal some red background colour). Only when I entered 10 AGAIN in the input prompt (current_hp was now 60), then the green bar was updated correctly as expected.

... (same as the block of code above)
window = pygame.display.set_mode((300, 300))
character = Player('player.png', [150, 150], window, 80)
character.draw()                    #1 additional
character.draw_health_bar(window)   #2 additional
pygame.display.flip()               #3 additional
run = True
while run:
... (same as the block of code above)

Only this arrangement of code would work perfectly as expected.

window = pygame.display.set_mode((300, 300))
character = Player('player.png', [150, 150], window, 80)
run = True
while run:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
    
    character.draw()
    character.draw_health_bar(window)
    pygame.display.flip() 
    
    damage = int(input('enter number:'))
    character.current_hp -= damage

This is so bewildering, I can’t understand it. Can someone please explain to me like I’m 5?