title: global variables in web2py
body: <p>I want to understand how global variables are implemented in web2py web framework.
  I assume that a reader knows the structure of web2py app.</p>
  
  <p>I don't understand how come variable <code>db</code> is available in every controller <code>.py</code> file without any import statement.
  I found that it <code>db</code> is created in <code>models/db.py</code> and it is bonded to <code>current.db</code>.
  How <code>db</code> is made globally available without any import.
  Thank you!</p>