title: Sum of specific key-value in python dictionary
body: <p>I have a <code>school</code> dictionary as follow- </p>
  
  <pre><code>{
      ID('6a15ce'): {
          'count': 5,
          'amount': 0,
          'r_amount': None,
          'sub': &lt; subobj &gt;
      }, ID('464ba1'): {
          'count': 2,
          'amount': 120,
          'r_amount': None,
          'sub': &lt; subobj2 &gt;
      }
  }
  </code></pre>
  
  <p>I want to find out the sum of <code>amount</code>  , doing as follow-</p>
  
  <p><code>{k:sum(v['amount']) for k,v in school.items()}</code></p>
  
  <p>but here I am getting error <code>TypeError: 'int' object is not iterable</code> what could be efficient way to achieve. </p>