title: Why doesn't Python 3 find installed packages (e.g. BeautifulSoup4)?
body: <p>Using Ubuntu 16.04 with Python 3.5 I get import errors for some packages, e.g. 'BeautifulSoup4' or 'requests'. Both libraries are installed from the Ubuntu repositories:</p>
  
  <pre><code>$ dpkg --get-selections | grep -E "python3-req|python3-bs"
  python3-bs4                         install
  python3-requests                    install
  </code></pre>
  
  <p>Yet I get "ImportError: no module named 'bs4'/'requests'". </p>
  
  <pre><code>$ python3 -c "import bs4"
  Traceback (most recent call last):
    File "&lt;string&gt;", line 1, in &lt;module&gt;
  ImportError: No module named 'bs4'
  </code></pre>
  
  <p>Only when I (additionally!) install those libraries via pip3 it works. The documentation of BeautifulSoup says to install it as "python3-bs4" via apt. Why does it only work with the pip3 method? What's the purpose of the python3-bs4/python3-requests packages?</p>