Source: 📖 Python Cookbook ch8.5 p250

Using __private attributes and methods

The use of double-underscore, or dunder, names signifies a private attribute or method. Names that are stated with two leading underscores belong only to the class in which they are declared—they are not inherited by sub-classes. The reason is that python renames these attributes behind the scenes. Take the following example:

class B:
	def __init__(self):
		self.__private = 0
		
	def __private_method(self):
		...
		
	def public_method(self):
		...

Here, python will rename __private as _B__private and __private_method as _B__private_method. The reason for this is so that these attributes cannot be overridden via inheritance—they belong only to the class in which they are declared. Consider the following:

class C(B):
	def __init__(self):
		super().__init__()
		self.__private = 1	# Does not override `B.__private`
		
	def __private_method(self):
		'''
		Does not override `B.__private_method()`
		'''
		...
		

In this case, the private names are renamed to _C__private and _C__private_method behind the scenes, differentiating them from the names of the methods in the B base class.

See also:

• Using _internal attributes and methods
• Pythonic naming conventions