title: Why is `source` used in the .bash_profile with virtualenvwrapper?
body: <p><code>virtualenvwrapper</code> instructs us to put <code>source /usr/local/bin/virtualenvwrapper.sh</code> in our <code>.bash_profile</code>. Why? </p>
  
  <p>I see it creates <code>.virtualenvs</code> the first time it's run. Is <code>.virtualenvs</code> newly created every time I <code>$ source .bash_profile</code>? If yes, is this so <code>.virtualenvs</code> can be updated as <code>virtualenvwrapper</code> is updated?</p>