What do I need to code for smooth jumping in pygame?

Question

I have looked at some other people's code but I can't seem to get it fixed. I want my jumps to be smooth and possibly use another image for going upwards and then falling back down. So it will look a little like an animation. Here's my code:

class GameState:

    def start(self):
        key = pygame.key.get_pressed()
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit()
                quit()
            
            if key[pygame.K_SPACE]:
                pass    #NEED HELP HERE
                   #If this happens I should also go to GS.state_manager('MAIN')
                   #to start the main game. This is like a starting screen
                    
                
        screen.blit(BG, (0,0))
        screen.blit(BIRD, (BIRD_rect.x, BIRD_rect.y))
        screen.blit(start_text, (start_text_rect.x, start_text_rect.y))
        clock.tick(30)
        pygame.display.flip()