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

Python eggs are zip-compressed packages containing both Python modules and metadata. The egg cache is where the the extracted contents of the egg are stored so that the Python modules contained within are usable.