Private copy of method in Python base class

[c0d3rX]

There is an example of code in Python documentation as follows:

class Mapping:
	def __init__(self, iterable):
		self.items_list = []
		self.__update(iterable)
	def update(self, iterable):
		for item in iterable:
			self.items_list.append(item)
	__update = update #private copy of original update () method

In the last line of code is the comment #private copy of original update(). According to the literature, this is supposed to create a private copy of the update method in the base class in case an update method is created in a sub class.

Does anyone have any insight on the significance of doing this?