title: How to use ignore-installed in python setup.py
body: <p>For a project of mine, i am using a forked project of pypi. As some people have the depedency already installed, my fork is only installed if the user does not have the dependency already installed.</p>
  
  <p>With pip you can use <code>--ignore-installed</code> to ignore the already existing dependency. </p>
  
  <p>How can i achieve the same in my <code>setup.py</code>?</p>
  
  <p>I tried</p>
  
  <pre class="lang-py prettyprint-override"><code>install_requires = ['some_package --ignore-installed']
  </code></pre>
  
  <p>but an error is raised when installing:</p>
  
  <pre><code>ERROR: error in dummy setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers; Invalid requirement, parse error at "'--ignore'"
  </code></pre>