mot=DummyMotor("motor")
reg= DummyRegister("register")
mot.initialize()
reg.initialize()
beam_ok = True



def is_beam_ok():
     return beam_ok

def wait_beam():
    if not is_beam_ok():        
        print "Waiting for beam..."
        while not is_beam_ok():
            time.sleep(0.1)
        print "Beam ok"


def before_read(pos, scan):
    wait_beam()
    #... 

def after_read(rec, scan):
    if not is_beam_ok():  
      print "Beam is down invalidating record ", rec.index, " at position ", rec.setpoints
      rec.invalidate()
      return 
    #... 


a= lscan(mot, reg, 0.0, 10.0, 0.1, latency=0.1, before_read=before_read, after_read=after_read)