title: How to define a sub-module in Python?
body: <p>Python has a module named "os". It also has some other module named "os.path" which is categorized under the "os".</p>
  
  <p>I can use "os.path" methods even if only import the "os" module.</p>
  
  <pre><code>import os
  
  print(os.path.join("sdfs","x"))
  </code></pre>
  
  <p>I wonder how can I define a sub-module like this?</p>