I am using Ubuntu 16.10,and I only installed virtualenv using aptitude. After creating a new environment,and listed installed packages using pip freeze I am always getting the output of
pkg-resources==0.0.0
According to http://stackoverflow.com/a/40167445/486919 and http://stackoverflow.com/a/40167000/486919,this is a bug on Ubuntu's end and a bug report has been made at https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1635463. Consider clicking on "Does this bug affect you?" followed by "Yes,it affects me" to bring this bug to the maintainers' attention.
For the time being,as stated in http://stackoverflow.com/a/40167445/486919,you can use pip freeze with grep -v to create requirements.txt that you can subsequently use with pip install -r:
pip freeze | grep -v "pkg-resources" > requirements.txt
https://askubuntu.com/questions/854249/pkg-resources-0-0-0-in-pip-freeze-bug/854254
