I&#39;ve just upgraded from Python 2.6.1 to 2.6.4 on my development machine and upon starting a python script was presented with the following message:

&gt; Can&#39;t extract file(s) to egg cache
&gt; 
&gt; The following error occurred while
&gt; trying to extract file(s) to  the
&gt; Python egg cache:
&gt; 
&gt; [Errno 13] Permission denied:
&gt; &#39;/var/www/.python-eggs&#39;
&gt; 
&gt; The Python egg cache directory is
&gt; currently set to:
&gt; 
&gt; /var/www/.python-eggs
&gt; 
&gt; Perhaps your account does not have
&gt; write access to this directory?   You
&gt; can change the cache directory by
&gt; setting the PYTHON_EGG_CACHE 
&gt; environment variable to point to an
&gt; accessible directory.

There isn&#39;t anything in the [python docs][1] so I&#39;m at a bit of a loss regarding best-practices on where to put this directory and what it&#39;s used for.

Can someone explain what the Python egg cache is?

Also, can you explain why/how it is different to the `site-packages` directory Python uses to store eggs (as I understand it)?


  [1]: http://docs.python.org/search.html?q=PYTHON_EGG_CACHE&amp;check_keywords=yes&amp;area=default

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What is the Python egg cache (PYTHON_EGG_CACHE)?