This avoids:
TypeError: in method 'comedi_get_cmd_generic_timed',
argument 5 of type 'unsigned int'
#construct a comedi command
cmd = c.comedi_cmd_struct()
-ret = c.comedi_get_cmd_generic_timed(dev,subdevice,cmd,nchans,1.0e9/freq)
+period = int(1.0e9/freq) # in nanoseconds
+ret = c.comedi_get_cmd_generic_timed(dev,subdevice,cmd,nchans,period)
if ret: raise "Error comedi_get_cmd_generic failed"
cmd.chanlist = mylist # adjust for our particular context