03 Jul 2010
神秘的__init__.py
Python中的module和package的作用不言而喻：Each module has its own private symbol table, which is used as the global symbol table by all functions defined in the module. Thus, the author of a module can use global variables in the module without worrying about accidental clashes with a user’s global variables.而Packages are a way of structuring Python’s module namespace by using “dotted module names”.
让package称为module那样的访问，__init__ .py 。一个文件夹A如果存在文件__init__.py(哪怕这个文件夹是空的)，就表明A不是一个简简单单的文件夹，而是一个包。
另外__init__.py控制着包的导入行为。比如现在存在这样一个pacakge：
Package1/
__init__.py
Module1.py
Module2.py
Package2/ __init__.py
Module1.py
Module2.py
如果执行语句：
import Package1
Package1.Module1
肯定会报错
AttributeError: ‘module’ object has no attribute ‘Module1’
为何？因为Module1只是Package1的一个submodule，并不是attribute，通过dir(package)就可以看出。如果想执行正确上面的语句，就必须在__init__.py中执行：
import Module1
另外，语句
from Package1 import *
到底import了那些，完全依靠__init__.py中的__all__属性。
另外，关于from和import需要注意的是：
Note that when using from package import item, the item can be either a submodule (or subpackage) of the package, or some other name defined in the package, like a function, class or variable. The import statement first tests whether the item is defined in the package; if not, it assumes it is a module and attempts to load it. If it fails to find it, an ImportError exception is raised.
Contrarily, when using syntax like import item.subitem.subsubitem, each item except for the last must be a package; the last item can be a module or a package but can’t be a class or function or variable defined in the previous item.
Resouces & References
- Modules
- [Python模块包中__init__.py文件的作用][]