title: Why does adding a trailing comma after an expression create a tuple?
body: <p>Why does adding a trailing comma after an expression create a <code>tuple</code> with the expression's value? E.g. in this code:</p>
  <pre><code>&gt;&gt;&gt; abc = 'mystring',
  &gt;&gt;&gt; print(abc)
  ('mystring',)
  </code></pre>
  <p>Why is the printed output <code>('mystring',)</code>, and not just <code>mystring</code>?</p>