title: Filling in values in python dictionary
body: <p>I have the following example code </p>
  
  <pre><code>test_dict = dict.fromkeys(['x','y','z'], np.zeros(5))
  
  for j in range(5):
      test_dict['x'][j]= j*10
  
  print(test_dict)  
  </code></pre>
  
  <p>I expect that this should fill in the value in 'x' as {0., 10., 20., 30., 40.} and it does. But it also fills in the same values for 'y' and 'z' which I don't want. Why is this happening and how can I prevent it? </p>