title: Sum of string values of a list in dictionary
body: <p>I am having trouble deriving the overall sum of string values.
  While I am able to generate the following if there are in integers form:</p>
  
  <pre><code>size = {
      'serverA' : [10, 10],
      'serverB' : [3, 3, 3],
  }
  </code></pre>
  
  <p>If I used <code>sums = {k: sum(i for i in v if isinstance(i, int)) for k, v in size.items()}</code>, I will be able to get output such as <code>{'serverA': 20, 'serverB': 9}</code></p>
  
  <p>However, instead of having integers in the values of the list, I got string as follows:</p>
  
  <pre><code>size = {
      'serverA' : ['10', '10'],
      'serverB' : ['3', '3', '3'],
  }
  </code></pre>
  
  <p>And if I used the same command to generate the sum of values, while there are no errors, the sums are clearly wrong in which the output is <code>{'serverA': 0, 'serverB': 0}</code></p>