title: Calculating strings as values
body: <p>Is it possible in Python to calculate a term in a string?
  For example:</p>
  
  <pre><code>string_a = "4 ** (3 - 2)"
  
  unknown_function(string_a) = 4
  </code></pre>
  
  <p>Is this possible? Is there a function that mimics "unknown_function" in my example?</p>