title: How to update a file in python egg
body: <p>We are using trac. In our setup we have a problem that is solved in repository. So I got the fixed file commit_update.py from the repository and I need to place it into Trac-0.12-py2.6.egg.</p>
  
  <p>As egg is just a ziped filed I just unziped it, changed the file and ziped again. After restarting trac, I've got a an error message:</p>
  
  <pre><code>ExtractionError: Can't extract file(s) to egg cache
  
  The following error occurred while trying to extract file(s) to the Python egg 
  cache:
  
      [Errno 20] Not a directory
  
  The Python egg cache directory is currently set to: 
  
      /var/trac/plugin-cache
  
  Perhaps your account does not have write access to this directory?  You can 
  change the cache directory by setting the PYTHON_EGG_CACHE environment
  variable to point to an accessible directory.
  </code></pre>
  
  <p>I don't quite understand why I got this error, because I'm running trac instance under www-data and it is the owner of the plugin-cache.</p>
  
  <p>Will appreciate any ideas.</p>