why does variable not change in function

im using python 3.10.2
i wanted to make a program that cycled thru red green and blue values but the variable is not changing allthough i made variables global before the for loop

i wrote this

r,g,b=255,0,0
def c(c,v):
    global r,g,b
    for _ in range(255):
        exec(f'{c}-=1')
        exec(f'{v}+=1')
        print(r,g,b)
c('r','g')
c('g','b')
c('b','r')

it prints this

255 0 0
255 0 0
255 0 0
255 0 0
255 0 0
255 0 0
...

(many more times than this)

i expected it to print this

255 0 0
254 1 0
253 2 0
252 3 0
251 4 0
250 5 0
...

what do i need to do
(sorry for bad english)

>Solution :

It seems that the global keyword declaration is not read by the exec function.

I think I got the wanted result doing this

(Also you could have a much simpler solution without using exec but I guess you have your reasons 🙂 )

r, g, b = 255, 0, 0


def c(c, v):
    for _ in range(255):
        exec(f'global {c} ; {c}-=1')
        exec(f'global {v}; {v}+=1')
        print(r, g, b)


c('r', 'g')
c('g', 'b')
c('b', 'r')