Issue with Local Variable in PyGame Drawing

Question

I am trying to draw some lines to make a box in pygame. My issue is I have a UnboundLocalVariable 'start_position' referenced before assignment. I sort of understand why it is happening because it is outside of the function's scope, but when I put it inside the function, it doesn't seem to set the proper coordinates. I believe turning this into a Class may be my best course of action but I was struggling to do that. If anyone has any suggestions on how to resolve this it would be much appreciated.

Here is the full code:

import pygame

#Intialize game
pygame.init()
#Setup screen dimensions and draw
WIDTH, HEIGHT = 1024, 1024
screen = pygame.display.set_mode((WIDTH, HEIGHT))
#Set global X,Y
X = 0
Y = 0

start_position = [X, Y]
position = [X, Y]
move_positive = + 50
move_negative = -50

def move_right():
    position[0] = position[0] + move_positive
    return position
def move_left():
    position[0] = position[0] + move_negative
    return position
def move_up():
    position[1] = position[1] + move_negative
    return position
def move_down():
    position[1] = position[1] + move_positive
    return position



def draw_square():
    
    #pygame.draw.line(surface, color, start_pos, end_pos, width)
    move_right()
    pygame.draw.line(screen, 'blue', start_position, position, 1) 
    start_position = position
    move_down()
    pygame.draw.line(screen, 'blue', start_position, position, 1)
    start_position = position
    move_left()
    pygame.draw.line(screen, 'blue', start_position, position, 1)
    start_position = position
    move_up()
    pygame.draw.line(screen, 'blue', start_position, position, 1)
    start_position = position


def main():

    while True:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit()
                exit()

        draw_square()
        pygame.display.update()
       

if __name__ == '__main__':
    main()

The issue is within the draw_square function. Thank you for your time.

score 1 · Answer 1 · answered May 27 '22 at 00:10

try this?

import pygame

#Intialize game
pygame.init()
#Setup screen dimensions and draw
WIDTH, HEIGHT = 1024, 1024
screen = pygame.display.set_mode((WIDTH, HEIGHT))
#Set global X,Y
X = 0
Y = 0

start_position = [X, Y]
position = [X, Y]
move_positive = + 50
move_negative = -50

def move_right():
    position[0] = position[0] + move_positive
    return position
def move_left():
    position[0] = position[0] + move_negative
    return position
def move_up():
    position[1] = position[1] + move_negative
    return position
def move_down():
    position[1] = position[1] + move_positive
    return position



def draw_square():
    global start_position #This was added

    #pygame.draw.line(surface, color, start_pos, end_pos, width)
    move_right()
    pygame.draw.line(screen, 'blue', start_position, position, 1) 
    start_position = position
    move_down()
    pygame.draw.line(screen, 'blue', start_position, position, 1)
    start_position = position
    move_left()
    pygame.draw.line(screen, 'blue', start_position, position, 1)
    start_position = position
    move_up()
    pygame.draw.line(screen, 'blue', start_position, position, 1)
    start_position = position


def main():

    while True:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit()
                exit()

        draw_square()
        pygame.display.update()
       

if __name__ == '__main__':
    main()

score 1 · Answer 2 · answered May 27 '22 at 00:27

I think this will work for you.

I experienced this a bit ago. To actually edit the values of start_position andposition, you need to use global <VARIABLE> to be able to do that. Simply add global start_position at the start of draw_square(). I hope this helps!

Issue with Local Variable in PyGame Drawing

2 Answers2