title: Ignoring some requirements when installing pip requirements
body: <p>I am using <code>requirements.txt</code> to install requirements for my virtualenv. I use ansible for deployments which installs requirements on remote hosts.</p>
  
  <p><strong>Problem:</strong></p>
  
  <ol>
  <li><p>Ignoring some requirements</p></li>
  <li><p>Ignoring already installed requirements (something like <code>pip freeze</code> and if the package shows up, don't install it and don't even upgrade)</p></li>
  </ol>
  
  <p><strong>Solutions according to me:</strong></p>
  
  <ol>
  <li><p>I can grep installed packages and make a <code>requirements2.txt</code> having only the ones needed. (Also, remove packages being installed from GIT)</p></li>
  <li><p>I don't understand what <code>--ignore-installed</code> would do in this case? </p></li>
  <li><p>Any other solution? </p></li>
  </ol>