title: Text formatting error: '=' alignment not allowed in string format specifier
body: <p>What does <code>'=' alignment</code> mean in the following error message, and why does this code cause it?</p>
  
  <pre class="lang-python prettyprint-override"><code>&gt;&gt;&gt; "{num:03}".format(num="1")
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  ValueError: '=' alignment not allowed in string format specifier
  </code></pre>
  
  <p>The code has a subtle problem: the input value <code>"1"</code> is text, not a number. But the error message doesn't appear to have anything to do with that.</p>
  
  <p>Nothing in the error message indicates why “'=' alignment” is relevant,
  and it does not appear in the code. So what is the significance of emitting that error message?</p>