How to use variables together with functions (2)

I'm just looking for an answer on how to count with a passable variable.
####################### This always returns 0 inside the loop ##
a = 0
def addOne(a):
    a +=1
while exists("1316568059480.png"): ## just made this to control the loop.
    addOne(a)
    popup(str(a)) ## need this to count up by one each click.
##################################################

########## This works as a counter but is useless##############
a = 0
while exists("1316568059480.png"): ## just made this to control the loop.
    a +=1
    popup(str(a)) ## need this to count up by one each click.
##################################################

#### This works just the same as the first example ##############
def program():
        a = 0
    def countOne(a):
        a +=1
    while exists("1316568059480.png"): ## just made this to control the loop.
        countOne(a)
        popup(str(a)) ## need this to count up by one each click.
program()
#################################################