A study drill at the end of the chapter asks me to make a function that calls a while loop that will count in an interval decided by the user, but I keep getting an endless loop. If I replace the x with a number, it will end, but if I leave it as x, as far as I can tell, it doesn't register the raw_input() from the user.
def counting_up():
    i = 0
    numbers = []
    print "Where do you want to end?"
    x = raw_input(">")
    print "How much would you like to increment by?"
    a = raw_input(">")
    while i < x:
        print "At the top, i is %d." % i
        numbers.append(i)
        i = i + a
        print "Numbers now: ", numbers
        print "At the bottom, i is %d." % i
        print "Your numbers: ", numbers
        for num in numbers:
            print num
counting_up()
 
     
    