Crashing Python, and need to check for ESC

Scripting
4

I do not know what RhinoPython.Host does, but you could just define the number of iterations, like:

import rhinoscriptsyntax as rs

print "This is a test"

ballx = 0 # Ball position variables
bally = 0
ballxmove = 0.3
ballymove = 0.7

i = 0
maxSteps = 200    # number of iterations
while i < maxSteps:

    ballx = ballx + ballxmove    # Update ball position
    bally = bally + ballymove
    
    sph = rs.AddSphere( (ballx,bally,0) ,2)
    rs.DeleteObject(sph)
    if ballx > 30:       # Ball reached screen edges?
        ballxmove = ballxmove * -1
    if ballx < 0:
        ballxmove = ballxmove* -1
    if bally > 30:
        ballymove = ballymove* -1
    if bally < 0:
        ballymove = ballymove* -1
    i += 1

Sorry if this was not of a help.