def empty ( self , start = None , stop = None ) : self . set ( NOT_SET , start = start , stop = stop )
python defeine set range
Empty the range from start to stop .
def empty(self, start=None, stop=None):
		"""Empty the range from start to stop.

		Like delete, but no Error is raised if the entire range isn't mapped.
		"""
		self.set(NOT_SET, start=start, stop=stop)