virtualenv and VIRTUAL_ENV keyword

Question

After having installed a new virtualenv, for example called ENV, if I type

. /path/to/ENV/bin/activate

python
import os
print os.environ['VIRTUAL_ENV']

Then I see the /path/to/ENV/

However, if I type

/path/to/ENV/bin/python

And then

import os
print os.environ['VIRTUAL_ENV']

I've got a key error So what is the fundamental difference between these two methods? Thanks,

Answer 1

Inside the script at bin/activate, there's a line that looks like this:

VIRTUAL_ENV="/Users/me/.envs/myenv"
export VIRTUAL_ENV

Which is what's responsible for setting your VIRTUAL_ENV environment variable. When you don't use activate, that variable never gets exported - so it's not present in os.environ.