问题描述
我从没注意到在今天之前在某些软件包上定义的__path__属性.根据文档:
I had never noticed the __path__ attribute that gets defined on some of my packages before today. According to the documentation:
虽然此功能并不常见 需要时，可以用来扩展 包中找到的一组模块.
While this feature is not often needed, it can be used to extend the set of modules found in a package.
有人可以向我解释这到底是什么意思，为什么我要使用它?
Could somebody explain to me what exactly this means and why I would ever want to use it?
推荐答案
这通常与 pkgutil一起使用以便将软件包放在磁盘上.例如，zope.interface和zope.schema是单独的发行版(zope是命名空间包").您可能在/usr/lib/python2.6/site-packages/zope/interface/中安装了zope.interface，而在/home/me/src/myproject/lib/python2.6/site-packages/zope/schema中更本地地使用了zope.schema.
This is usually used with pkgutil to let a package be laid out across the disk. E.g., zope.interface and zope.schema are separate distributions (zope is a "namespace package"). You might have zope.interface installed in /usr/lib/python2.6/site-packages/zope/interface/, while you are using zope.schema more locally in /home/me/src/myproject/lib/python2.6/site-packages/zope/schema.
如果将pkgutil.extend_path(__path__, __name__)放在/usr/lib/python2.6/site-packages/zope/__init__.py中，则zope.interface和zope.schema都是可导入的，因为pkgutil将__path__更改为['/usr/lib/python2.6/site-packages/zope', '/home/me/src/myproject/lib/python2.6/site-packages/zope'].
If you put pkgutil.extend_path(__path__, __name__) in /usr/lib/python2.6/site-packages/zope/__init__.py then both zope.interface and zope.schema will be importable because pkgutil will have change __path__ to ['/usr/lib/python2.6/site-packages/zope', '/home/me/src/myproject/lib/python2.6/site-packages/zope'].
pkg_resources.declare_namespace(Setuptools的一部分)与pkgutil.extend_path类似，但更了解路径上的压缩.
pkg_resources.declare_namespace (part of Setuptools) is like pkgutil.extend_path but is more aware of zips on the path.
手动更改__path__并不常见，也可能没有必要，尽管在调试名称空间包的导入问题时查看变量很有用.
Manually changing __path__ is uncommon and probably not necessary, though it is useful to look at the variable when debugging import problems with namespace packages.
您也可以使用__path__进行猴子补丁，例如，有时我会通过创建文件distutils/__init__.py来创建猴子补丁distutils，而该文件早于sys.path:
You can also use __path__ for monkeypatching, e.g., I have monkeypatched distutils at times by creating a file distutils/__init__.py that is early on sys.path:
import os
stdlib_dir = os.path.dirname(os.__file__)
real_distutils_path = os.path.join(stdlib_dir, 'distutils')
__path__.append(real_distutils_path)
execfile(os.path.join(real_distutils_path, '__init__.py'))
# and then apply some monkeypatching here...
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