Object moving in a spiral

[JakeWitten]

I'm having a hard time figuring out how to get an object (a ball in this case) to move in a spiral.  I figured the best way to go about this would be to use polar coordinates for x and z, but I can't seem to implement it properly.  Any ideas?

from visual import *

myuniv = display(range = vector(25,25,25))

theta = 1
r = 2
s = vector(r*cos(theta),2,r*sin(theta))  
v = vector(theta,0,theta)
a = vector(0,-9.8,0)

dt = 0.1
t = 0.0

ball = sphere(pos = s, radius = 0.25, color = color.cyan)

while s.y > -100.0:
    rate(25)
    theta = theta + 1
    s = s+ v*dt
    v = v + a*dt
    t = t + dt
    ball.pos = s
    ballghost = sphere(pos = s, radius = 0.2)

Am I on the right track here?  I'm very new to python and coding languages in general.

Thanks!