title: What does a b prefix before a python string mean?
body: <p>In a python source code I stumbled upon I've seen a small <strong>b</strong> before a string like in:</p>
  
  <pre><code>b"abcdef"
  </code></pre>
  
  <p>I know about the <strong><code>u</code></strong> prefix signifying a unicode string, and the <strong><code>r</code></strong> prefix for a raw string literal. </p>
  
  <p>What does the <code>b</code> stand for and in which kind of source code is it useful as it seems to be exactly like a plain string without any prefix?</p>