title: Python : defaultdict every value is updated
body: <p>I'm working for more than one hour on a stupid problem but i can't figure out the solution.
  I create a defaultdict(list) with an initial list and update those list through a for loop.
  However, everytime i update one value, all other values are updated with same value. 
  Can somebody help me please ? 
  Here is my code :</p>
  
  <pre><code>from collections import defaultdict
  base = ["coucou", "salut", "tchao"]
  initial_vector = [0]*len(base)
  dict_vectorized_documents = defaultdict(lambda: initial_vector)
  inversed_index = {"coucou": [(1, 3), (100, 4)], "salut": [(1, 1), (99, 2), (33, 3)], "tchao": [(1, 5)]}
  
  for i, word in enumerate(base):
  print(word)
  for element in inversed_index[word]:
      print(element[0])
      print(i)
      print(element[1])
      print(dict_vectorized_documents[element[0]][i])
      dict_vectorized_documents[element[0]][i] = element[1]
      print(dict_vectorized_documents)
  
  print(dict_vectorized_documents)
  </code></pre>
  
  <p>And here is my logs when i run it : </p>
  
  <pre><code>coucou
  1
  0
  3
  0
  defaultdict(&lt;function &lt;lambda&gt; at 0x7fcc5fac1f28&gt;, {1: [3, 0, 0]})
  100
  0
  4
  3
  defaultdict(&lt;function &lt;lambda&gt; at 0x7fcc5fac1f28&gt;, {1: [4, 0, 0], 100:      [4, 0, 0]})
  salut
  1
  1
  1
  0
  defaultdict(&lt;function &lt;lambda&gt; at 0x7fcc5fac1f28&gt;, {1: [4, 1, 0], 100: [4, 1, 0]})
  99
  1
  2
  1
  defaultdict(&lt;function &lt;lambda&gt; at 0x7fcc5fac1f28&gt;, {1: [4, 2, 0], 99: [4, 2, 0], 100: [4, 2, 0]})
  33
  1
  3
  2
  defaultdict(&lt;function &lt;lambda&gt; at 0x7fcc5fac1f28&gt;, {1: [4, 3, 0], 99: [4, 3, 0], 100: [4, 3, 0], 33: [4, 3, 0]})
  tchao
  1
  2
  5
  0
  </code></pre>
  
  <p>Thank you very much ! </p>