title: What is the design reason for the ordering of Python packages path?
body: <p>I <a href="https://stackoverflow.com/q/14882377/656912">understand</a> that, on OS X, the default ordering for searching paths for packages is (in the form reported by <code>pprint(sys.path)</code>) something like:</p>
  
  <pre><code>['',
   '/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']
  </code></pre>
  
  <p>For example, on my system (OS 10.9) I have:</p>
  
  <pre><code>['',
  '/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']
  </code></pre>
  
  <p>Other systems seem to adopt a similar structure.</p>
  
  <p>What is the logic of this ordering, in particular why are "eggs" sought first, and why is <code>site-packages</code> used last? </p>