2 different lines of code doing the same thing returns a different output

June 26, 2023

So I was trying to implement basic movement into my "game" when I ran into an issue which I ended up solving, but not understanding why my previous solution didn’t work. At first I tried doing pygame.draw.rect(screen, character_color, character) where "character" was character = pygame.Rect(character_pos_y, character_pos_x, character_size_y, character_size_x) and it didn’t move. then I decided to do pygame.draw.rect(screen, character_color, (character_pos_y, character_pos_x, character_size_y, character_size_x)) and it worked. And it started working.

When I ran pygame.draw.rect(screen, character_color, character) my character position was changing in code, but not on the screen. Anyone knows why my previous solution didn’t work?

import pygame
import time
pygame.init()
#defining
ground_pos_x = 0
ground_pos_y = 400
ground_size_x = 500
ground_size_y = 50
ground_color = (0, 0, 0)
ground = pygame.Rect(ground_pos_x, ground_pos_y, ground_size_x, ground_size_y)
character_pos_x = 200
character_pos_y = 200
character_size_y = 100
character_size_x = 100
character_color = (50, 50, 50)
character = pygame.Rect(character_pos_y, character_pos_x, character_size_y, character_size_x)
fps = 60
clock = pygame.time.Clock()
background_colour = (111, 222, 111)
screen = pygame.display.set_mode((500, 500))

pygame.display.set_caption("test")
screen.fill(background_colour)

pygame.draw.rect(screen, character_color, character)
pygame.draw.rect(screen, ground_color, ground)
pygame.display.update()
running = True
while running:
    clock.tick(fps)
    screen.fill(background_colour)
    #movement
    keys = pygame.key.get_pressed()
    if keys[pygame.K_UP]:
        character_pos_x -= 10
    if keys[pygame.K_LEFT]:
        character_pos_y -= 10
    if keys[pygame.K_DOWN]:
        character_pos_x += 10
    if keys[pygame.K_RIGHT]:
        character_pos_y += 10

    pygame.draw.rect(screen, character_color, (character_pos_y, character_pos_x, character_size_y, character_size_x))
    pygame.draw.rect(screen, ground_color, ground)

    #if character.colliderect(ground):
    #    print("works")
    #else:
    #    print("nope")

    pygame.display.update()
    for event in pygame.event.get():

        if event.type == pygame.QUIT:
            running = False

>Solution :

The position stored in character does not magically change when you change character_pos_x or character_pos_y. character is not somehow tied to character_pos_x and character_pos_y. You have to recreate the pygame.Rect object after changing character_pos_x or character_pos_y:

while running:
    # [...]

    keys = pygame.key.get_pressed()
    if keys[pygame.K_UP]:
        character_pos_x -= 10
    if keys[pygame.K_LEFT]:
        character_pos_y -= 10
    if keys[pygame.K_DOWN]:
        character_pos_x += 10
    if keys[pygame.K_RIGHT]:
        character_pos_y += 10

    character = pygame.Rect(character_pos_y, character_pos_x, character_size_y, character_size_x)
    pygame.draw.rect(screen, character_color, character)

Alternatively, you can also change character.x and character.y instead of character_pos_x or character_pos_y:

while running:
    # [...]

    keys = pygame.key.get_pressed()
    if keys[pygame.K_UP]:
        character.x -= 10
    if keys[pygame.K_LEFT]:
        character.y -= 10
    if keys[pygame.K_DOWN]:
        character.x += 10
    if keys[pygame.K_RIGHT]:
        character.y += 10

    pygame.draw.rect(screen, character_color, character)

Also note that your code can be simplified (also see How can I make a sprite move when key is held down):

while running:
    # [...]

    keys = pygame.key.get_pressed()
    character.x += (keys[pygame.K_RIGHT] - [pygame.K_LEFT]) * 10
    character.y += (keys[pygame.K_DOWN] - [pygame.K_UP]) * 10
    pygame.draw.rect(screen, character_color, character)