What is the Python egg cache (PYTHON_EGG_CACHE)?

Question

I'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:

Can't extract file(s) to egg cache

The following error occurred while trying to extract file(s) to the Python egg cache:

[Errno 13] Permission denied: '/var/www/.python-eggs'

The Python egg cache directory is currently set to:

/var/www/.python-eggs

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.

There isn't anything in the python docs so I'm at a bit of a loss regarding best-practices on where to put this directory and what it'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)?

Answer 1

From my investigations it turns out that some eggs are packaged as zip files, and are saved as such in Python's site-packages directory.

These zipped eggs need to be unzipped before they can be executed, so are expanded into the PYTHON_EGG_CACHE directory which by default is ~/.python-eggs (located in the user's home directory). If this doesn't exist it causes problems when trying to run applications.

There are a number of fixes:

1. Create a .python-eggs directory in the user's home directory and make it writable for the user.
2. Create a global directory for unzipping (eg. /tmp/python-eggs) and set the environment variable PYTHON_EGG_CACHE to this directory.
3. Use the -Z switch when using easy_install to unzip the package when installing.