title: Difference between pip install options "ignore-installed" and "force-reinstall"
body: <p>There are two <code>pip install</code> options related to reinstalling the packages, which are <code>--ignore-installed</code> and <code>--force-reinstall</code>.</p>
  
  <p>These two options described as following in the official doc</p>
  
  <pre><code>--force-reinstall
  Reinstall all packages even if they are already up-to-date.
  
  -I, --ignore-installed
  Ignore the installed packages (reinstalling instead).
  </code></pre>
  
  <p>It seems that they all ignore something and do the reinstallation but I cannot tell the difference between them (I can see some difference if I actually execute them ... but I cannot explain). If I search "force reinstall packages in pip", the result lists both <code>--ignore-installed</code> and <code>--force-reinstall</code>, which confuses me for a long time.</p>