moving object forward backward

[Zword]

That is not working. This is the error I get:

Traceback (most recent call last):
File "C:\Program Files (x86)\WorldViz\Vizard30/python\vizact.py", line 3019, in __onkeydown
self._callGroup(self.__keydownmap[key])
File "C:\Program Files (x86)\WorldViz\Vizard30/python\vizact.py", line 2971, in _callGroup
val = e.call(arg)
File "C:\Program Files (x86)\WorldViz\Vizard30/python\vizact.py", line 2794, in _callStatic
return func(*args,**kwargs)
File "Test wall.py", line 39, in moveForward
pos += 0.5
UnboundLocalError: local variable 'pos' referenced before assignment


My code:

def moveForward():
	object = viz.pick()
	pos += 0.5
	if object.valid():
		object.setPosition([pos,0,0])
		viz.callback(viz.COLLIDE_BEGIN_EVENT,oncollide)

vizact.onkeydown( 'w', moveForward