single and double underscore of class member name in Python

Single underscore

_single_leading_underscore: weak “internal use” indicator. E.g. from M import * does not import objects whose name starts with an underscore. However, nothing special is done with the name itself.

_foo: this is just a convention, a way for the programmer to indicate that the variable is private.

Double underscore

Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped.

__foo: this has real meaning, the interpreter replaces this name with _classname__foo as a way to ensure that the name will not overlap with a similar name in another class.

__foo__: this is just a convention, a way for the Python system to use names that won’t conflict with user names.

Example

>>> class MyClass():
...     def __init__(self):
...         self.__superprivate = "Hello"
...         self._semiprivate = ", world!"
...
>>> mc = MyClass()
>>> print mc.__superprivate
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: myClass instance has no attribute '__superprivate'
>>> print mc._semiprivate
, world!
>>> print mc.__dict__
{'_MyClass__superprivate': 'Hello', '_semiprivate': ', world!'}

Summary

• _var: 下划线开头的变量或方法只在内部使用，与Java不同，Python在“私有”和“公共”变量之间并没有很强的区别。
• var_: 某个变量最合适的名称已被Python语言中的关键字占用，可以追加一个下划线来绕过命名冲突。
• __var: 也称为名称改写（name mangling），双下划线前缀会让Python解释器重写属性名称，以避免子类中的命名冲突。
• __var__: 双下划线方法通常被称为魔法方法，参考下表。
• _: 循环中不需要访问运行的索引可以使用_来表示临时值；还能表示REPL会话中上一个表达式的结果。

Other magic methods in Python

Reference

https://rszalski.github.io/magicmethods