1 一个目录要成为一个package必须有__init__.py文件
The __init__.py files are required to make Python treat the directories as containing packages; this is done to prevent directories with a common name, such as string, from unintentionally hiding valid modules that occur later (deeper) on the module search path. In the simplest case, __init__.py can just be an empty file, but it can also execute initialization code for the package or set the __all__ variable, described later.
2 __init__.py文件中的内容
2.1 该文件可以是空的
2.2 该文件中也可以定义类和函数
假如目录名是AAA，那么要使用该package中的类和函数，直接import AAA即可。
另外，要用该目录下的module BBB，直接from AAA import BBB即可。