title: Why does `pip list` not show all Python packages installed by `apt` on Debian / Ubuntu?
body: <p>On Debian based Linux distributions <code>pip</code> usually detects Python packages installed by <code>apt</code> (in <code>/usr/lib/pythonX/dist-packages</code>). However, this is not valid for <code>PyQt5</code> for example (<code>PyQt5</code> does not show up when running <code>pip list</code> or <code>pip freeze</code>). Does anyone know why?</p>
  
  <p>With Docker you can run the following steps to reproduce the problem:</p>
  
  <ul>
  <li><p>Run a Debian unstable:</p>
  
  <pre><code>docker run -it debian:unstable
  </code></pre></li>
  <li><p>Install <code>python3-pip</code> and <code>python3-pyqt5</code>:</p>
  
  <pre><code>apt update
  apt install --no-install-recommends python3-pip python3-pyqt5
  </code></pre></li>
  <li><p>Run</p>
  
  <pre><code>pip3 list
  </code></pre>
  
  <p>Only <code>pip</code> will be listed.</p></li>
  <li><p>Install <code>python3-requests</code></p>
  
  <pre><code>apt install --no-install-recommends python3-requests
  </code></pre>
  
  <p>and run</p>
  
  <pre><code>pip3 list
  </code></pre>
  
  <p>again. The <code>requests</code> package will be listed.</p></li>
  </ul>