title: how prevent errors with look ahead
body: <p>Recursively into a loop, i need to extract the value of a specific key from different dictionaries called into the loop.
  The key register might be present or not inside the retrieved dictionary. When not present, my query to such key , return an error: <code>KeyError: 'c'</code></p>
  <pre><code>dict2 ={'a':[1,2,3,4],'b':[5,6,7,8]}
  dict1 ={'a':[1,2,3,4],'b':[5,6,7,8],'c':[9,9,1,3]}
  lis = [dict1,dict2]
  for tmp in lis:
      my_var = tmp['c']
  </code></pre>
  <p>At 1st loop, I correctly obtain the vector <code>[9,9,1,3]</code>; at the second the error.</p>
  <p>Which could be a built in function to evaluate ahead <code>tmp['c']</code> before execute it and get the error ?  thks</p>