title: Sum a list of numbers in Python
body: <p>Given a list of numbers such as:</p>
  <pre><code>[1, 2, 3, 4, 5, ...]
  </code></pre>
  <p>How do I calculate their total sum:</p>
  <pre><code>1 + 2 + 3 + 4 + 5 + ...
  </code></pre>
  <p>How do I calculate their pairwise averages:</p>
  <pre><code>[(1+2)/2, (2+3)/2, (3+4)/2, (4+5)/2, ...]
  </code></pre>