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After the upgrade of your Ubuntu (maybe from 12.04 to 14.04) as you saw there is a difference in python versions. This means that your virtual environments which try to use old one has broken. You need to recreate all of your virtual environments, because some strange conflicts may happens even in the standard python libraries.
EDIT:
If you use virtualenvwrapper you can type:
rmvirtualenv <envname>
mkvirtualenv <envname> -r <path/to/requirement.txt>
This will remove and create environment again with packages listed in your requirements.txt
file.
NOTE: If you are in active environment first deactivate it before removing virtual environemnt.
Source:stackexchange.com