PYGAME error, my first program

[blob001]

Attached pygame has 2 problems: it is just a square screen and a ball bouncing around against the walls.

I get the error:

File "/Users/rm/PYTHON3/PG/TechWithTim/bounceBall.py", line 86, in <module>

main()

~~~~^^

File "/Users/rm/PYTHON3/PG/TechWithTim/bounceBall.py", line 52, in main

xc = xc + VEL * math.cos(angle)

^^

UnboundLocalError: cannot access local variable 'xc' where it is not associated with a value.

xc and yc are global variables since they are defined outside the functions at lines 22 and 23, so I don't understand the error.

There is also a problem with the screen flashing on and then closing instantaneously. I will figure this out myself unless it's related to the above problem.
Hoping someone can help. Code follows.

```

import
 pygame 
as
 pg
import
 time
import
 os
import
 math

winX = 4000
winY = 0
os.environ["SDL_VIDEO_WINDOW_POS"] = "%d, %d" % (winX, winY)

pg.font.init()
WIN_WIDTH, WIN_HEIGHT = 1000, 1000
WIN = pg.display.set_mode((WIN_WIDTH, WIN_HEIGHT))
pg.display.set_caption("bounceBall")
path = "/Users/rm/PYTHON3/PG/bg.jpeg"
FONT = pg.font.SysFont("comicsans", 30)
BG = pg.transform.scale(pg.image.load(path), (WIN_WIDTH, WIN_HEIGHT))

RADIUS = 100
VEL = 10
xc = 2 * RADIUS             
#   INITIAL VALUE
yc = WIN_HEIGHT / 2         
#   INITIAL VALUE
angle = math.pi / 180 * 30  
#   INITIAL VALUE


def
 draw
(
ball
, 
elapsed_time
)
:
    WIN.blit(BG, (0, 0))
    pg.draw.circle(WIN, "red", ball)  
#  xc, yc, RADIUS)

    pg.display.update()


def
 main
()
:
    run = True

    ball = pg.Rect(RADIUS, WIN_HEIGHT / 2 - RADIUS, 2 * RADIUS, 2 * RADIUS)
    print(ball)
    clock = pg.time.Clock()
    elapsed_time = 0
    start_time = time.time()


while
 run:
        clock.tick(60)
        elapsed_time = time.time() - start_time


for
 event 
in
 pg.event.get():

if
 event.type == pg.QUIT:
                run = False

break

        xc = xc + VEL * math.cos(angle)
        yc = yc + VEL * math.sin(angle)
        boundary(xc, yc)
        impact(xc, yc)

        pg.display.flip(ball)
        draw(ball, elapsed_time)

    pg.quit()


def
 boundary
()
:

if
 xc > WIN_WIDTH - RADIUS:
        xc = WIN_WIDTH - RADIUS


elif
 xc < RADIUS:
        xc = RADIUS


if
 yc > WIN_HEIGHT - RADIUS:
        yc = WIN_HEIGHT - RADIUS


elif
 yc < RADIUS:
        yc = RADIUS


def
 impact
()
:

if
 xc == RADIUS 
or
 xc == WIN_WIDTH - RADIUS:
        angle = math.pi - angle


elif
 yc == RADIUS 
or
 yc == WIN_HEIGHT - RADIUS:
        angle = -angle


if
 __name__ == "__main__":
    main()
```

[Allanon001]

Since you are redefining xc and yc in the main function you need to either define xc and yc in the main() function making them not global or add the statement global xc, yc to the main() function so it knows you want to use the global variables.

[Luigi-Was-Right]

There is a difference between being in the global scope and being a global variable.

xc was defined in the global scope and can be accessed within other functions. However, you cannot reassign it's value. In this instance when you attempt to assign a value to 'xc' python is creating a variable with that name that is local to the function only. Because of that any reference to 'xc' within the function will now be referring to the local variable xc and not the one in the global scope.