Python single underline, double underline, and tail and bottom double underline description:

Single underline, double underline, and tail-to-bottom double underline description:

• __foo__ : Defines a special method, typically a system-defined name, similar to   __init__ ()   and so on.

• _foo : A variable of type protected that begins with a single underscore, that is, the protection type can only allow access to its own and subclasses, not to   from module import *

• __foo : A double underscore is a variable of private type, which is allowed to be accessed only by the class itself.

Source: http://www.runoob.com/python/python-object.html

From for notes (Wiz)

Python single underline, double underline, and tail and bottom double underline description: