title: String format with optional dict key-value
body: <p>Is there any way to format string with dict but optionally without key errors?</p>
  
  <p>This works fine:</p>
  
  <pre><code>opening_line = '%(greetings)s  %(name)s !!!'
  opening_line % {'greetings': 'hello', 'name': 'john'}
  </code></pre>
  
  <p>But let's say I don't know the name, and I would like to format above line 
  only for <code>'greetings'</code>. Something like,</p>
  
  <pre><code> opening_line % {'greetings': 'hello'}
  </code></pre>
  
  <p>Output would be fine even if:</p>
  
  <pre><code>'hii %(name)s !!!'  # keeping name un-formatted 
  </code></pre>
  
  <p>But this gives <code>KeyError</code> while unpacking</p>
  
  <p>Is there any way?</p>