title: partial string formatting
body: <p>Is it possible to do partial string formatting with the advanced string formatting methods, similar to the string template <code>safe_substitute()</code> function?</p>
  
  <p>For example:</p>
  
  <pre><code>s = '{foo} {bar}'
  s.format(foo='FOO') #Problem: raises KeyError 'bar'
  </code></pre>