title: How can I use a pip requirements file to uninstall as well as install packages?
body: <p>I have a pip requirements file that changes during development.</p>
  
  <p>Can <code>pip</code> be made to <strong>uninstall</strong> packages that do not appear in the requirements file as well as installing those that do appear? Is there a standard method?</p>
  
  <p>This would allow the pip requirements file to be the canonical list of packages - an 'if and only if' approach.</p>
  
  <p><strong>Update</strong>: I suggested it as a new feature at <a href="https://github.com/pypa/pip/issues/716">https://github.com/pypa/pip/issues/716</a></p>