1._*:The special identifier _ is used in the interactive interpreter to store the result of the last evaluation; it is stored in the builtins module. When not in interactive mode, _ has no special meaning and is not defined
2.__*__系统定义的名称,
3.__*类的私有属性和方法
参考:
https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers