title: Python: Create list of lists as dictionary values
body: <p>I have two lists that I would like to associate by index as key value pairs in a dictionary. The key list has multiple identical elements. I would like the all elements in the value list to be paired as a list of list. I am using the list.append() method, but this is not giving me the desired output. Any recommendations on the code or should I be looking at the problem in a different way?</p>
  
  <pre><code>list1 = ['a', 'b', 'b', 'b', 'c']
  list2 = [['1', '2', '3'], ['4', '5', '6'], [ '7', '8', '9'], ['10', '11', '12'], ['13', '14', '15']]
  
  combo = {}
  for i in range(len(list1)):
      if list1[i] in combo:
          combo[list1[i]].append(list2[i])
      else:
          combo[list1[i]] = list2[i]
  </code></pre>
  
  <p>Current output:</p>
  
  <pre><code>{'a': ['1', '2', '3'], 'b': ['4', '5', '6', [ '7', '8', '9'], ['10', '11', '12']], 'c': ['13', '14', 15']}
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>{'a': [['1', '2', '3']], 'b': [['4', '5', '6'], [ '7', '8', '9'], ['10', '11', '12']], 'c': [['13', '14', 15']]}
  </code></pre>