title: Are nested format specifications legal?
body: <p>Recently, I came across the following oddity.  Nesting <code>{}</code>-enclosed format fields seems to work in both Python 2.7 and 3.6, but I can't find anything in the docs to say that should be so.  For example, I get the following result on both 3.6 and 2.7:</p>
  
  <pre><code>&gt;&gt;&gt; '{:{}.{}f}'.format(27.5, 6, 2)
  ' 27.50'
  </code></pre>
  
  <p>Has anyone seen this before, and is it an intended feature?  Normally, I'd dismiss this as an implementation quirk, and maybe report it as a bug. Two things, though:  Python docs don't always put all the information in the place I'd look for it, and this is a Very Nice Feature to have. </p>