title: Set or update dictionary entries code pattern in Python
body: <p>Given a dict with the following format:</p>
  
  <pre><code>dict_list = {key0: [list0,list1], key1: [list0,list1], ...}
  </code></pre>
  
  <p>I am currently updating <code>dict_list</code> in the following manner:</p>
  
  <pre><code>for key in list_of_possibly_new_keys:
     if key not in dict_list.keys():
        dict_list[key] = [list0, list1]
     else:
        dict_list[key][0].extend(list0)
        dict_list[key][1].extend(list1)
  </code></pre>
  
  <p>Is there a way to make this code shorter using native constructs (i.e. no additional methods)?  I am increasingly encountering this data "set or update" pattern in my code, and wondering whether there is a more concise way to do this.</p>
  
  <p>UPDATE/CLARIFICATION:</p>
  
  <p>Sample actual contents of dict_list:</p>
  
  <pre><code>dict_list = {'John Solver': [['05/10/2013','05/14/2013','05/22/2013'],[20.33,40.12,10.13]]}
  </code></pre>
  
  <p>Where the floats could represent John Solver's daily expenses in US dollars.</p>
  
  <p>If possible, we'd like to keep using two long lists instead of many date-expense pairs, as we'd like to conveniently perform a sum of expenses operation using sum(dict_list['John Solver'][1]).</p>