title: Why do I have to add a blank __init__.py file to import from subdirectories
body: <p>I can import from subdirectories with a blank <code>__init__.py</code> file. That works fine. I think that init file in python works like an index file. But I really don't understand why the <code>__init__.py</code> file is blank to import. How do <code>__init__.py</code> file and importing really work?</p>
  
  <p>What have to be <code>__init__.py</code> files content?</p>