title: virtualenv and VIRTUAL_ENV keyword
body: <p>After having installed a new virtualenv, for example called ENV, if I type</p>
  
  <pre><code>. /path/to/ENV/bin/activate
  
  python
  import os
  print os.environ['VIRTUAL_ENV']
  </code></pre>
  
  <p>Then I see the <code>/path/to/ENV/</code></p>
  
  <p>However, if I type</p>
  
  <pre><code>/path/to/ENV/bin/python
  </code></pre>
  
  <p>And then</p>
  
  <pre><code>import os
  print os.environ['VIRTUAL_ENV']
  </code></pre>
  
  <p>I've got a key error
  So what is the fundamental difference between these two methods?
  Thanks,</p>