title: Optional variables in Python
body: <p>do you guys know if there exists a Python equivalent to boost::optional in C++ (or std::optional since C++11: <a href="http://en.cppreference.com/w/cpp/utility/optional" rel="noreferrer">http://en.cppreference.com/w/cpp/utility/optional</a>), i.e. a library that handles <em>semantically optional</em> variables? </p>
  
  <p>I know how to implement it myself or use the other solutions (like <code>foo = (bar, True)</code> which I find ugly and unreadable). Just curious if there is an existing solution.</p>