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Detailed explanation of the role of __init__.py file in Python
Detailed explanation of the role of __init__.py file in Python
__init__.py 文件的作用是将文件夹变为一个Python模块,Python 中的每个模块的包中，都有__init__.py 文件。
通常__init__.py 文件为空，但是我们还可以为它增加其他的功能。我们在导入一个包时，实际上是导入了它的__init__.py文件。这样我们可以在__init__.py文件中批量导入我们所需要的模块，而不再需要一个一个的导入。
# package # __init__.py import re import urllib import sys import os # a.py import package print(package.re, package.urllib, package.sys, package.os)
注意这里访问__init__.py文件中的引用文件，需要加上包名。
__init__.py中还有一个重要的变量，__all__, 它用来将模块全部导入。
# __init__.py __all__ = ['os', 'sys', 're', 'urllib'] # a.py from package import *
这时就会把注册在__init__.py文件中__all__列表中的模块和包导入到当前文件中来。
可以了解到，__init__.py主要控制包的导入行为。要想清楚理解__init__.py文件的作用，还需要详细了解一下import语句引用机制：
可以被import语句导入的对象是以下类型：
•模块文件（.py文件）
•C或C++扩展（已编译为共享库或DLL文件）
•包（包含多个模块）
•内建模块（使用C编写并已链接到Python解释器中）
当导入模块时，解释器按照sys.path列表中的目录顺序来查找导入文件。
import sys >>> print(sys.path) # Linux: ['', '/usr/local/lib/python3.4', '/usr/local/lib/python3.4/plat-sunos5', '/usr/local/lib/python3.4/lib-tk', '/usr/local/lib/python3.4/lib-dynload', '/usr/local/lib/python3.4/site-packages'] # Windows: ['', 'C:\\WINDOWS\\system32\\python34.zip', 'C:\\Documents and Settings\\weizhong', 'C:\\Python34\\DLLs', 'C:\\Python34\\lib', 'C:\\Python34\\lib\\plat-win', 'C:\\Python34\\lib\\lib-tk', 'C:\\Python34\\Lib\\site-packages\\pythonwin', 'C:\\Python34', 'C:\\Python34\\lib\\site-packages', 'C:\\Python34\\lib\\site-packages\\win32', 'C:\\Python34\\lib\\site-packages\\win32\\lib', 'C:\\Python34\\lib\\site-packages\\wx-2.6-msw-unicode']
其中list第一个元素空字符串代表当前目录。
关于.pyc 文件 与 .pyo 文件
.py文件的汇编,只有在import语句执行时进行，当.py文件第一次被导入时，它会被汇编为字节代码，并将字节码写入同名的.pyc文件中。后来每次导入操作都会直接执行.pyc 文件（当.py文件的修改时间发生改变，这样会生成新的.pyc文件），在解释器使用-O选项时，将使用同名的.pyo文件，这个文件去掉了断言（assert）、断行号以及其他调试信息，体积更小，运行更快。（使用-OO选项，生成的.pyo文件会忽略文档信息）
导入模块
模块通常为单独的.py文件，可以用import直接引用，可以作为模块的文件类型有.py、.pyo、.pyc、.pyd、.so、.dll
在导入模块时，解释器做以下工作：
1.已导入模块的名称创建新的命名空间，通过该命名空间就可以访问导入模块的属性和方法。
2.在新创建的命名空间中执行源代码文件。
3.创建一个名为源代码文件的对象，该对象引用模块的名字空间，这样就可以通过这个对象访问模块中的函数及变量
import 语句可以在程序的任何位置使用，你可以在程序中多次导入同一个模块，但模块中的代码仅仅在该模块被首次导入时执行。后面的import语句只是简单的创建一个到模块名字空间的引用而已。
sys.modules字典中保存着所有被导入模块的模块名到模块对象的映射。
导入包
多个相关联的模块组成一个包，以便于维护和使用，同时能有限的避免命名空间的冲突。一般来说，包的结构可以是这样的：
package |- subpackage1 |- __init__.py |- a.py |- subpackage2 |- __init__.py |- b.py
有以下几种导入方式：
import subpackage1.a # 将模块subpackage.a导入全局命名空间，例如访问a中属性时用subpackage1.a.attr from subpackage1 import a # 将模块a导入全局命名空间，例如访问a中属性时用a.attr_a from subpackage.a import attr_a # 将模块a的属性直接导入到命名空间中，例如访问a中属性时直接用attr_a 使用from语句可以把模块直接导入当前命名空间，from语句并不引用导入对象的命名空间，而是将被导入对象直接引入当前命名空间。
以上所述是小编给大家介绍的Python中__init__.py文件的作用详解，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对PHP中文网的支持！
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