title: How to explain the first colon in Python slice syntax list[::-1]
body: <p>I recently read a code snippets about how to reverse a sequence</p>
  
  <pre><code>&gt;&gt; l = [1,2,3,4,5,6]
  &gt;&gt; print l[::-1]
  </code></pre>
  
  <h3>Output</h3>
  
  <pre><code>&gt;&gt; [6,5,4,3,2,1]
  </code></pre>
  
  <p>How to explain the first colon in bracket?</p>