title: Can I get mapping-keys?
body: <p>If I in python got the string <code>Hello %(firstname)s</code>, can I get the expected mapping-keys in that string?</p>
  <p>Some example pseudo code:</p>
  <pre><code>&gt;&gt;&gt; s = 'Hello %(name)s'
  &gt;&gt;&gt; s%{'name':'Ulf'}
  'Hello Ulf'
  &gt;&gt;&gt; # The thing below is roughly what I want.
  &gt;&gt;&gt; s.get_map_keys()
  ['name']
  &gt;&gt;&gt; 'Hello %(name)s %(family)s'.get_map_keys()
  ['name', 'family']
  </code></pre>
  <p>NB: I know that this is old-style formatting and that there is better better ways of producing string-templates. But I'm analyzing existing code and do not have control over the strings, I just need to process them and see what replacements are in use.</p>