How to activate/deactivate a virtualenv from python code?
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For activation there is a script that activates a virtualenv from an already running python interpeter using `execfile('C:/path/to/virtualev/Scripts/activate_this.py', dict(__file__='C:/path/to/virtualev/Scripts/activate_this.py'))`. However since I can still import packages that are not in the virtualenv from the current python script I am confused about how it works.  
For deactivation there is no python script at all.  
What should I do?