title: Search for a value in a nested dictionary python
body: <p>Search  for a value and get the parent dictionary names (keys):</p>
  
  <pre><code>Dictionary = {dict1:{
          'part1': {
              '.wbxml': 'application/vnd.wap.wbxml',
              '.rl': 'application/resource-lists+xml',    
          },
          'part2':
              {'.wsdl': 'application/wsdl+xml',
              '.rs': 'application/rls-services+xml',
              '.xop': 'application/xop+xml',
              '.svg': 'image/svg+xml',
              },
          'part3':{...}, ...
  
     dict2:{
            'part1': {    '.dotx': 'application/vnd.openxmlformats-..'                           
              '.zaz': 'application/vnd.zzazz.deck+xml',
              '.xer': 'application/patch-ops-error+xml',}  
            },
            'part2':{...},
            'part3':{...},...  
  
      },...
  </code></pre>
  
  <p>In above dictionary I need to search values like: <code>"image/svg+xml"</code>. Where, none of the values are repeated in the dictionary. How to search the <code>"image/svg+xml"</code>? so that it should return the parent keys in a dictionary <code>{ dict1:"part2" }</code>.</p>
  
  <p><strong>Please note</strong>: Solutions should work <em>unmodified</em> for <em>both</em> Python 2.7 and Python 3.3.</p>