title: Update dict without adding new keys?
body: <p>What is a good 1-liner (other than putting this into a function) to achieve this:</p>
  
  <pre><code># like dict1.update(dict2) but does not add any keys from dict2
  # that are not already in dict1
  for k in dict1:
      if k in dict2:
          dict1[k]=dict2[k]
  </code></pre>
  
  <p>I guess that this would work, but uses a "protected" function:</p>
  
  <pre><code>[dict1.__setitem__(k, dict2[k]) for k in dict1 if k in dict2]
  </code></pre>