title: Why is ''.join() faster than += in Python?
body: <p>I'm able to find a bevy of information online (on Stack&nbsp;Overflow and otherwise) about how it's a very inefficient and bad practice to use <code>+</code> or <code>+=</code> for concatenation in Python.</p>
  
  <p>I can't seem to find WHY <code>+=</code> is so inefficient. Outside of a mention <a href="https://stackoverflow.com/a/1350289/3903011" title="What is the most efficient string concatenation method in python?">here</a> that "it's been optimized for 20% improvement in certain cases" (still not clear what those cases are), I can't find any additional information.</p>
  
  <p>What is happening on a more technical level that makes <code>''.join()</code> superior to other Python concatenation methods?</p>