我understand，在OS X上，搜索包路径的默认顺序是（以pprint(sys.path)报告的形式），例如：
['',
'/Library/Python/X.X/site-packages/PKGS_WITH_EGGS.egg',
...
'/System/Library/Frameworks/Python.framework/Versions/X.X/lib/pythonX.X/SOME_PLATFORM_PKGS,
...
'/System/Library/Frameworks/Python.framework/Versions/X.X/Extras/lib/python',
...
'/System/Library/Frameworks/Python.framework/Versions/X.X/lib/pythonX.X/OTHER_PLATFORM_PKGS,
...
'/System/Library/Frameworks/Python.framework/Versions/X.X/Extras/lib/python/PyObjC',
'/Library/Python/X.X/site-packages']
例如，在我的系统（OS 10.9）上，我有：
['',
'/Library/Python/2.7/site-packages/astropy-0.3.dev2837-py2.7-macosx-10.8-intel.egg',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python27.zip',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-darwin',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac/lib-scriptpackages',
'/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-old',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload',
'/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/PyObjC',
'/Library/Python/2.7/site-packages']
其他系统似乎采用了类似的结构。
这种排序的逻辑是什么，特别是为什么首先寻找“蛋”，为什么最后使用site-packages？