How do I "pretend" to install a package using pip?
<p>This feels like such a simple question, but I can't find any reference in the <a href="https://pip.pypa.io/en/stable/" rel="nofollow noreferrer">pip documentation</a> and <a href="https://stackoverflow.com/questions/11460681/listing-the-dependencies-of-a-package-using-pip">the only question that seemed relevant</a> mentions a flag that has apparently been deprecated since version 1.5 (version 8.1 is current at the time of this writing).</p>

<p>How do I "pretend" to install a package or list of packages using pip, without actually installing them?  I have two separate use cases for this:</p>

<ul>
<li>I need to see what packages out of a long (~70 line) requirements.txt are missing, <em>without actually installing them</em>; seeing what requirements are already satisfied without installing the missing requirements would satisfy this for me.</li>
<li>Finding the dependencies for a package that I have not yet installed on my computer, without using something like Portage or Aptitude.</li>
</ul>